diff --git a/src/applications/files/management/PhabricatorFilesManagementCatWorkflow.php b/src/applications/files/management/PhabricatorFilesManagementCatWorkflow.php
index ed354d4a89..896165f585 100644
--- a/src/applications/files/management/PhabricatorFilesManagementCatWorkflow.php
+++ b/src/applications/files/management/PhabricatorFilesManagementCatWorkflow.php
@@ -1,39 +1,55 @@
 <?php
 
 final class PhabricatorFilesManagementCatWorkflow
   extends PhabricatorFilesManagementWorkflow {
 
   protected function didConstruct() {
     $this
       ->setName('cat')
       ->setSynopsis(
         pht('Print the contents of a file.'))
       ->setArguments(
         array(
+          array(
+            'name' => 'begin',
+            'param' => 'bytes',
+            'help' => pht('Begin printing at a specific offset.'),
+          ),
+          array(
+            'name' => 'end',
+            'param' => 'bytes',
+            'help' => pht('End printing at a specific offset.'),
+          ),
           array(
             'name'      => 'names',
             'wildcard'  => true,
           ),
         ));
   }
 
   public function execute(PhutilArgumentParser $args) {
     $console = PhutilConsole::getConsole();
 
     $names = $args->getArg('names');
     if (count($names) > 1) {
       throw new PhutilArgumentUsageException(
         pht('Specify exactly one file to print, like "F123".'));
     } else if (!$names) {
       throw new PhutilArgumentUsageException(
         pht('Specify a file to print, like "F123".'));
     }
 
     $file = head($this->loadFilesWithNames($names));
 
-    echo $file->loadFileData();
+    $begin = $args->getArg('begin');
+    $end = $args->getArg('end');
+
+    $iterator = $file->getFileDataIterator($begin, $end);
+    foreach ($iterator as $data) {
+      echo $data;
+    }
 
     return 0;
   }
 
 }
diff --git a/src/applications/files/storage/PhabricatorFile.php b/src/applications/files/storage/PhabricatorFile.php
index 446fbbbbe8..2fbcf3a891 100644
--- a/src/applications/files/storage/PhabricatorFile.php
+++ b/src/applications/files/storage/PhabricatorFile.php
@@ -1,1313 +1,1338 @@
 <?php
 
 /**
  * Parameters
  * ==========
  *
  * When creating a new file using a method like @{method:newFromFileData}, these
  * parameters are supported:
  *
  *   | name | Human readable filename.
  *   | authorPHID | User PHID of uploader.
  *   | ttl | Temporary file lifetime, in seconds.
  *   | viewPolicy | File visibility policy.
  *   | isExplicitUpload | Used to show users files they explicitly uploaded.
  *   | canCDN | Allows the file to be cached and delivered over a CDN.
  *   | mime-type | Optional, explicit file MIME type.
  *   | builtin | Optional filename, identifies this as a builtin.
  *
  */
 final class PhabricatorFile extends PhabricatorFileDAO
   implements
     PhabricatorApplicationTransactionInterface,
     PhabricatorTokenReceiverInterface,
     PhabricatorSubscribableInterface,
     PhabricatorFlaggableInterface,
     PhabricatorPolicyInterface,
     PhabricatorDestructibleInterface {
 
   const ONETIME_TEMPORARY_TOKEN_TYPE = 'file:onetime';
   const STORAGE_FORMAT_RAW  = 'raw';
 
   const METADATA_IMAGE_WIDTH  = 'width';
   const METADATA_IMAGE_HEIGHT = 'height';
   const METADATA_CAN_CDN = 'canCDN';
   const METADATA_BUILTIN = 'builtin';
   const METADATA_PARTIAL = 'partial';
 
   protected $name;
   protected $mimeType;
   protected $byteSize;
   protected $authorPHID;
   protected $secretKey;
   protected $contentHash;
   protected $metadata = array();
   protected $mailKey;
 
   protected $storageEngine;
   protected $storageFormat;
   protected $storageHandle;
 
   protected $ttl;
   protected $isExplicitUpload = 1;
   protected $viewPolicy = PhabricatorPolicies::POLICY_USER;
   protected $isPartial = 0;
 
   private $objects = self::ATTACHABLE;
   private $objectPHIDs = self::ATTACHABLE;
   private $originalFile = self::ATTACHABLE;
 
   public static function initializeNewFile() {
     $app = id(new PhabricatorApplicationQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withClasses(array('PhabricatorFilesApplication'))
       ->executeOne();
 
     $view_policy = $app->getPolicy(
       FilesDefaultViewCapability::CAPABILITY);
 
     return id(new PhabricatorFile())
       ->setViewPolicy($view_policy)
       ->setIsPartial(0)
       ->attachOriginalFile(null)
       ->attachObjects(array())
       ->attachObjectPHIDs(array());
   }
 
   protected function getConfiguration() {
     return array(
       self::CONFIG_AUX_PHID => true,
       self::CONFIG_SERIALIZATION => array(
         'metadata' => self::SERIALIZATION_JSON,
       ),
       self::CONFIG_COLUMN_SCHEMA => array(
         'name' => 'text255?',
         'mimeType' => 'text255?',
         'byteSize' => 'uint64',
         'storageEngine' => 'text32',
         'storageFormat' => 'text32',
         'storageHandle' => 'text255',
         'authorPHID' => 'phid?',
         'secretKey' => 'bytes20?',
         'contentHash' => 'bytes40?',
         'ttl' => 'epoch?',
         'isExplicitUpload' => 'bool?',
         'mailKey' => 'bytes20',
         'isPartial' => 'bool',
       ),
       self::CONFIG_KEY_SCHEMA => array(
         'key_phid' => null,
         'phid' => array(
           'columns' => array('phid'),
           'unique' => true,
         ),
         'authorPHID' => array(
           'columns' => array('authorPHID'),
         ),
         'contentHash' => array(
           'columns' => array('contentHash'),
         ),
         'key_ttl' => array(
           'columns' => array('ttl'),
         ),
         'key_dateCreated' => array(
           'columns' => array('dateCreated'),
         ),
         'key_partial' => array(
           'columns' => array('authorPHID', 'isPartial'),
         ),
       ),
     ) + parent::getConfiguration();
   }
 
   public function generatePHID() {
     return PhabricatorPHID::generateNewPHID(
       PhabricatorFileFilePHIDType::TYPECONST);
   }
 
   public function save() {
     if (!$this->getSecretKey()) {
       $this->setSecretKey($this->generateSecretKey());
     }
     if (!$this->getMailKey()) {
       $this->setMailKey(Filesystem::readRandomCharacters(20));
     }
     return parent::save();
   }
 
   public function getMonogram() {
     return 'F'.$this->getID();
   }
 
   public static function readUploadedFileData($spec) {
     if (!$spec) {
       throw new Exception('No file was uploaded!');
     }
 
     $err = idx($spec, 'error');
     if ($err) {
       throw new PhabricatorFileUploadException($err);
     }
 
     $tmp_name = idx($spec, 'tmp_name');
     $is_valid = @is_uploaded_file($tmp_name);
     if (!$is_valid) {
       throw new Exception('File is not an uploaded file.');
     }
 
     $file_data = Filesystem::readFile($tmp_name);
     $file_size = idx($spec, 'size');
 
     if (strlen($file_data) != $file_size) {
       throw new Exception('File size disagrees with uploaded size.');
     }
 
     self::validateFileSize(strlen($file_data));
 
     return $file_data;
   }
 
   public static function newFromPHPUpload($spec, array $params = array()) {
     $file_data = self::readUploadedFileData($spec);
 
     $file_name = nonempty(
       idx($params, 'name'),
       idx($spec,   'name'));
     $params = array(
       'name' => $file_name,
     ) + $params;
 
     return self::newFromFileData($file_data, $params);
   }
 
   public static function newFromXHRUpload($data, array $params = array()) {
     self::validateFileSize(strlen($data));
     return self::newFromFileData($data, $params);
   }
 
   private static function validateFileSize($size) {
     $limit = PhabricatorEnv::getEnvConfig('storage.upload-size-limit');
     if (!$limit) {
       return;
     }
 
     $limit = phutil_parse_bytes($limit);
     if ($size > $limit) {
       throw new PhabricatorFileUploadException(-1000);
     }
   }
 
 
   /**
    * Given a block of data, try to load an existing file with the same content
    * if one exists. If it does not, build a new file.
    *
    * This method is generally used when we have some piece of semi-trusted data
    * like a diff or a file from a repository that we want to show to the user.
    * We can't just dump it out because it may be dangerous for any number of
    * reasons; instead, we need to serve it through the File abstraction so it
    * ends up on the CDN domain if one is configured and so on. However, if we
    * simply wrote a new file every time we'd potentially end up with a lot
    * of redundant data in file storage.
    *
    * To solve these problems, we use file storage as a cache and reuse the
    * same file again if we've previously written it.
    *
    * NOTE: This method unguards writes.
    *
    * @param string  Raw file data.
    * @param dict    Dictionary of file information.
    */
   public static function buildFromFileDataOrHash(
     $data,
     array $params = array()) {
 
     $file = id(new PhabricatorFile())->loadOneWhere(
       'name = %s AND contentHash = %s LIMIT 1',
       self::normalizeFileName(idx($params, 'name')),
       self::hashFileContent($data));
 
     if (!$file) {
       $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
       $file = PhabricatorFile::newFromFileData($data, $params);
       unset($unguarded);
     }
 
     return $file;
   }
 
   public static function newFileFromContentHash($hash, array $params) {
     // Check to see if a file with same contentHash exist
     $file = id(new PhabricatorFile())->loadOneWhere(
       'contentHash = %s LIMIT 1',
       $hash);
 
     if ($file) {
       // copy storageEngine, storageHandle, storageFormat
       $copy_of_storage_engine = $file->getStorageEngine();
       $copy_of_storage_handle = $file->getStorageHandle();
       $copy_of_storage_format = $file->getStorageFormat();
       $copy_of_byte_size = $file->getByteSize();
       $copy_of_mime_type = $file->getMimeType();
 
       $new_file = PhabricatorFile::initializeNewFile();
 
       $new_file->setByteSize($copy_of_byte_size);
 
       $new_file->setContentHash($hash);
       $new_file->setStorageEngine($copy_of_storage_engine);
       $new_file->setStorageHandle($copy_of_storage_handle);
       $new_file->setStorageFormat($copy_of_storage_format);
       $new_file->setMimeType($copy_of_mime_type);
       $new_file->copyDimensions($file);
 
       $new_file->readPropertiesFromParameters($params);
 
       $new_file->save();
 
       return $new_file;
     }
 
     return $file;
   }
 
   public static function newChunkedFile(
     PhabricatorFileStorageEngine $engine,
     $length,
     array $params) {
 
     $file = PhabricatorFile::initializeNewFile();
 
     $file->setByteSize($length);
 
     // TODO: We might be able to test the first chunk in order to figure
     // this out more reliably, since MIME detection usually examines headers.
     // However, enormous files are probably always either actually raw data
     // or reasonable to treat like raw data.
     $file->setMimeType('application/octet-stream');
 
     $chunked_hash = idx($params, 'chunkedHash');
     if ($chunked_hash) {
       $file->setContentHash($chunked_hash);
     } else {
       // See PhabricatorChunkedFileStorageEngine::getChunkedHash() for some
       // discussion of this.
       $seed = Filesystem::readRandomBytes(64);
       $hash = PhabricatorChunkedFileStorageEngine::getChunkedHashForInput(
         $seed);
       $file->setContentHash($hash);
     }
 
     $file->setStorageEngine($engine->getEngineIdentifier());
     $file->setStorageHandle(PhabricatorFileChunk::newChunkHandle());
     $file->setStorageFormat(self::STORAGE_FORMAT_RAW);
     $file->setIsPartial(1);
 
     $file->readPropertiesFromParameters($params);
 
     return $file;
   }
 
   private static function buildFromFileData($data, array $params = array()) {
 
     if (isset($params['storageEngines'])) {
       $engines = $params['storageEngines'];
     } else {
       $size = strlen($data);
       $engines = PhabricatorFileStorageEngine::loadStorageEngines($size);
 
       if (!$engines) {
         throw new Exception(
           pht(
             'No configured storage engine can store this file. See '.
             '"Configuring File Storage" in the documentation for '.
             'information on configuring storage engines.'));
       }
     }
 
     assert_instances_of($engines, 'PhabricatorFileStorageEngine');
     if (!$engines) {
       throw new Exception(pht('No valid storage engines are available!'));
     }
 
     $file = PhabricatorFile::initializeNewFile();
 
     $data_handle = null;
     $engine_identifier = null;
     $exceptions = array();
     foreach ($engines as $engine) {
       $engine_class = get_class($engine);
       try {
         list($engine_identifier, $data_handle) = $file->writeToEngine(
           $engine,
           $data,
           $params);
 
         // We stored the file somewhere so stop trying to write it to other
         // places.
         break;
       } catch (PhabricatorFileStorageConfigurationException $ex) {
         // If an engine is outright misconfigured (or misimplemented), raise
         // that immediately since it probably needs attention.
         throw $ex;
       } catch (Exception $ex) {
         phlog($ex);
 
         // If an engine doesn't work, keep trying all the other valid engines
         // in case something else works.
         $exceptions[$engine_class] = $ex;
       }
     }
 
     if (!$data_handle) {
       throw new PhutilAggregateException(
         'All storage engines failed to write file:',
         $exceptions);
     }
 
     $file->setByteSize(strlen($data));
     $file->setContentHash(self::hashFileContent($data));
 
     $file->setStorageEngine($engine_identifier);
     $file->setStorageHandle($data_handle);
 
     // TODO: This is probably YAGNI, but allows for us to do encryption or
     // compression later if we want.
     $file->setStorageFormat(self::STORAGE_FORMAT_RAW);
 
     $file->readPropertiesFromParameters($params);
 
     if (!$file->getMimeType()) {
       $tmp = new TempFile();
       Filesystem::writeFile($tmp, $data);
       $file->setMimeType(Filesystem::getMimeType($tmp));
     }
 
     try {
       $file->updateDimensions(false);
     } catch (Exception $ex) {
       // Do nothing
     }
 
     $file->save();
 
     return $file;
   }
 
   public static function newFromFileData($data, array $params = array()) {
     $hash = self::hashFileContent($data);
     $file = self::newFileFromContentHash($hash, $params);
 
     if ($file) {
       return $file;
     }
 
     return self::buildFromFileData($data, $params);
   }
 
   public function migrateToEngine(PhabricatorFileStorageEngine $engine) {
     if (!$this->getID() || !$this->getStorageHandle()) {
       throw new Exception(
         "You can not migrate a file which hasn't yet been saved.");
     }
 
     $data = $this->loadFileData();
     $params = array(
       'name' => $this->getName(),
     );
 
     list($new_identifier, $new_handle) = $this->writeToEngine(
       $engine,
       $data,
       $params);
 
     $old_engine = $this->instantiateStorageEngine();
     $old_identifier = $this->getStorageEngine();
     $old_handle = $this->getStorageHandle();
 
     $this->setStorageEngine($new_identifier);
     $this->setStorageHandle($new_handle);
     $this->save();
 
     $this->deleteFileDataIfUnused(
       $old_engine,
       $old_identifier,
       $old_handle);
 
     return $this;
   }
 
   private function writeToEngine(
     PhabricatorFileStorageEngine $engine,
     $data,
     array $params) {
 
     $engine_class = get_class($engine);
 
     $data_handle = $engine->writeFile($data, $params);
 
     if (!$data_handle || strlen($data_handle) > 255) {
       // This indicates an improperly implemented storage engine.
       throw new PhabricatorFileStorageConfigurationException(
         "Storage engine '{$engine_class}' executed writeFile() but did ".
         "not return a valid handle ('{$data_handle}') to the data: it ".
         "must be nonempty and no longer than 255 characters.");
     }
 
     $engine_identifier = $engine->getEngineIdentifier();
     if (!$engine_identifier || strlen($engine_identifier) > 32) {
       throw new PhabricatorFileStorageConfigurationException(
         "Storage engine '{$engine_class}' returned an improper engine ".
         "identifier '{$engine_identifier}': it must be nonempty ".
         "and no longer than 32 characters.");
     }
 
     return array($engine_identifier, $data_handle);
   }
 
 
   public static function newFromFileDownload($uri, array $params = array()) {
     // Make sure we're allowed to make a request first
     if (!PhabricatorEnv::getEnvConfig('security.allow-outbound-http')) {
       throw new Exception('Outbound HTTP requests are disabled!');
     }
 
     $uri = new PhutilURI($uri);
 
     $protocol = $uri->getProtocol();
     switch ($protocol) {
       case 'http':
       case 'https':
         break;
       default:
         // Make sure we are not accessing any file:// URIs or similar.
         return null;
     }
 
     $timeout = 5;
 
     list($file_data) = id(new HTTPSFuture($uri))
         ->setTimeout($timeout)
         ->resolvex();
 
     $params = $params + array(
       'name' => basename($uri),
     );
 
     return self::newFromFileData($file_data, $params);
   }
 
   public static function normalizeFileName($file_name) {
     $pattern = "@[\\x00-\\x19#%&+!~'\$\"\/=\\\\?<> ]+@";
     $file_name = preg_replace($pattern, '_', $file_name);
     $file_name = preg_replace('@_+@', '_', $file_name);
     $file_name = trim($file_name, '_');
 
     $disallowed_filenames = array(
       '.'  => 'dot',
       '..' => 'dotdot',
       ''   => 'file',
     );
     $file_name = idx($disallowed_filenames, $file_name, $file_name);
 
     return $file_name;
   }
 
   public function delete() {
     // We want to delete all the rows which mark this file as the transformation
     // of some other file (since we're getting rid of it). We also delete all
     // the transformations of this file, so that a user who deletes an image
     // doesn't need to separately hunt down and delete a bunch of thumbnails and
     // resizes of it.
 
     $outbound_xforms = id(new PhabricatorFileQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withTransforms(
         array(
           array(
             'originalPHID' => $this->getPHID(),
             'transform'    => true,
           ),
         ))
       ->execute();
 
     foreach ($outbound_xforms as $outbound_xform) {
       $outbound_xform->delete();
     }
 
     $inbound_xforms = id(new PhabricatorTransformedFile())->loadAllWhere(
       'transformedPHID = %s',
       $this->getPHID());
 
     $this->openTransaction();
       foreach ($inbound_xforms as $inbound_xform) {
         $inbound_xform->delete();
       }
       $ret = parent::delete();
     $this->saveTransaction();
 
     $this->deleteFileDataIfUnused(
       $this->instantiateStorageEngine(),
       $this->getStorageEngine(),
       $this->getStorageHandle());
 
     return $ret;
   }
 
 
   /**
    * Destroy stored file data if there are no remaining files which reference
    * it.
    */
   public function deleteFileDataIfUnused(
     PhabricatorFileStorageEngine $engine,
     $engine_identifier,
     $handle) {
 
     // Check to see if any files are using storage.
     $usage = id(new PhabricatorFile())->loadAllWhere(
       'storageEngine = %s AND storageHandle = %s LIMIT 1',
       $engine_identifier,
       $handle);
 
     // If there are no files using the storage, destroy the actual storage.
     if (!$usage) {
       try {
         $engine->deleteFile($handle);
       } catch (Exception $ex) {
         // In the worst case, we're leaving some data stranded in a storage
         // engine, which is not a big deal.
         phlog($ex);
       }
     }
   }
 
 
   public static function hashFileContent($data) {
     return sha1($data);
   }
 
   public function loadFileData() {
 
     $engine = $this->instantiateStorageEngine();
     $data = $engine->readFile($this->getStorageHandle());
 
     switch ($this->getStorageFormat()) {
       case self::STORAGE_FORMAT_RAW:
         $data = $data;
         break;
       default:
         throw new Exception('Unknown storage format.');
     }
 
     return $data;
   }
 
+
+  /**
+   * Return an iterable which emits file content bytes.
+   *
+   * @param int Offset for the start of data.
+   * @param int Offset for the end of data.
+   * @return Iterable Iterable object which emits requested data.
+   */
+  public function getFileDataIterator($begin = null, $end = null) {
+    // The default implementation is trivial and just loads the entire file
+    // upfront.
+    $data = $this->loadFileData();
+
+    if ($begin !== null && $end !== null) {
+      $data = substr($data, $begin, ($end - $begin));
+    } else if ($begin !== null) {
+      $data = substr($data, $begin);
+    } else if ($end !== null) {
+      $data = substr($data, 0, $end);
+    }
+
+    return array($data);
+  }
+
+
   public function getViewURI() {
     if (!$this->getPHID()) {
       throw new Exception(
         'You must save a file before you can generate a view URI.');
     }
 
     return $this->getCDNURI(null);
   }
 
   private function getCDNURI($token) {
     $name = phutil_escape_uri($this->getName());
 
     $parts = array();
     $parts[] = 'file';
     $parts[] = 'data';
 
     // If this is an instanced install, add the instance identifier to the URI.
     // Instanced configurations behind a CDN may not be able to control the
     // request domain used by the CDN (as with AWS CloudFront). Embedding the
     // instance identity in the path allows us to distinguish between requests
     // originating from different instances but served through the same CDN.
     $instance = PhabricatorEnv::getEnvConfig('cluster.instance');
     if (strlen($instance)) {
       $parts[] = '@'.$instance;
     }
 
     $parts[] = $this->getSecretKey();
     $parts[] = $this->getPHID();
     if ($token) {
       $parts[] = $token;
     }
     $parts[] = $name;
 
     $path = '/'.implode('/', $parts);
 
     // If this file is only partially uploaded, we're just going to return a
     // local URI to make sure that Ajax works, since the page is inevitably
     // going to give us an error back.
     if ($this->getIsPartial()) {
       return PhabricatorEnv::getURI($path);
     } else {
       return PhabricatorEnv::getCDNURI($path);
     }
   }
 
   /**
    * Get the CDN URI for this file, including a one-time-use security token.
    *
    */
   public function getCDNURIWithToken() {
     if (!$this->getPHID()) {
       throw new Exception(
         'You must save a file before you can generate a CDN URI.');
     }
 
     return $this->getCDNURI($this->generateOneTimeToken());
   }
 
 
   public function getInfoURI() {
     return '/'.$this->getMonogram();
   }
 
   public function getBestURI() {
     if ($this->isViewableInBrowser()) {
       return $this->getViewURI();
     } else {
       return $this->getInfoURI();
     }
   }
 
   public function getDownloadURI() {
     $uri = id(new PhutilURI($this->getViewURI()))
       ->setQueryParam('download', true);
     return (string) $uri;
   }
 
   private function getTransformedURI($transform) {
     $parts = array();
     $parts[] = 'file';
     $parts[] = 'xform';
 
     $instance = PhabricatorEnv::getEnvConfig('cluster.instance');
     if (strlen($instance)) {
       $parts[] = '@'.$instance;
     }
 
     $parts[] = $transform;
     $parts[] = $this->getPHID();
     $parts[] = $this->getSecretKey();
 
     $path = implode('/', $parts);
     $path = $path.'/';
 
     return PhabricatorEnv::getCDNURI($path);
   }
 
   public function getProfileThumbURI() {
     return $this->getTransformedURI('thumb-profile');
   }
 
   public function getThumb60x45URI() {
     return $this->getTransformedURI('thumb-60x45');
   }
 
   public function getThumb160x120URI() {
     return $this->getTransformedURI('thumb-160x120');
   }
 
   public function getPreview100URI() {
     return $this->getTransformedURI('preview-100');
   }
 
   public function getPreview220URI() {
     return $this->getTransformedURI('preview-220');
   }
 
   public function getThumb220x165URI() {
     return $this->getTransfomredURI('thumb-220x165');
   }
 
   public function getThumb280x210URI() {
     return $this->getTransformedURI('thumb-280x210');
   }
 
   public function isViewableInBrowser() {
     return ($this->getViewableMimeType() !== null);
   }
 
   public function isViewableImage() {
     if (!$this->isViewableInBrowser()) {
       return false;
     }
 
     $mime_map = PhabricatorEnv::getEnvConfig('files.image-mime-types');
     $mime_type = $this->getMimeType();
     return idx($mime_map, $mime_type);
   }
 
   public function isAudio() {
     if (!$this->isViewableInBrowser()) {
       return false;
     }
 
     $mime_map = PhabricatorEnv::getEnvConfig('files.audio-mime-types');
     $mime_type = $this->getMimeType();
     return idx($mime_map, $mime_type);
   }
 
   public function isTransformableImage() {
     // NOTE: The way the 'gd' extension works in PHP is that you can install it
     // with support for only some file types, so it might be able to handle
     // PNG but not JPEG. Try to generate thumbnails for whatever we can. Setup
     // warns you if you don't have complete support.
 
     $matches = null;
     $ok = preg_match(
       '@^image/(gif|png|jpe?g)@',
       $this->getViewableMimeType(),
       $matches);
     if (!$ok) {
       return false;
     }
 
     switch ($matches[1]) {
       case 'jpg';
       case 'jpeg':
         return function_exists('imagejpeg');
         break;
       case 'png':
         return function_exists('imagepng');
         break;
       case 'gif':
         return function_exists('imagegif');
         break;
       default:
         throw new Exception('Unknown type matched as image MIME type.');
     }
   }
 
   public static function getTransformableImageFormats() {
     $supported = array();
 
     if (function_exists('imagejpeg')) {
       $supported[] = 'jpg';
     }
 
     if (function_exists('imagepng')) {
       $supported[] = 'png';
     }
 
     if (function_exists('imagegif')) {
       $supported[] = 'gif';
     }
 
     return $supported;
   }
 
   public function instantiateStorageEngine() {
     return self::buildEngine($this->getStorageEngine());
   }
 
   public static function buildEngine($engine_identifier) {
     $engines = self::buildAllEngines();
     foreach ($engines as $engine) {
       if ($engine->getEngineIdentifier() == $engine_identifier) {
         return $engine;
       }
     }
 
     throw new Exception(
       "Storage engine '{$engine_identifier}' could not be located!");
   }
 
   public static function buildAllEngines() {
     $engines = id(new PhutilSymbolLoader())
       ->setType('class')
       ->setConcreteOnly(true)
       ->setAncestorClass('PhabricatorFileStorageEngine')
       ->selectAndLoadSymbols();
 
     $results = array();
     foreach ($engines as $engine_class) {
       $results[] = newv($engine_class['name'], array());
     }
 
     return $results;
   }
 
   public function getViewableMimeType() {
     $mime_map = PhabricatorEnv::getEnvConfig('files.viewable-mime-types');
 
     $mime_type = $this->getMimeType();
     $mime_parts = explode(';', $mime_type);
     $mime_type = trim(reset($mime_parts));
 
     return idx($mime_map, $mime_type);
   }
 
   public function getDisplayIconForMimeType() {
     $mime_map = PhabricatorEnv::getEnvConfig('files.icon-mime-types');
     $mime_type = $this->getMimeType();
     return idx($mime_map, $mime_type, 'fa-file-o');
   }
 
   public function validateSecretKey($key) {
     return ($key == $this->getSecretKey());
   }
 
   public function generateSecretKey() {
     return Filesystem::readRandomCharacters(20);
   }
 
   public function updateDimensions($save = true) {
     if (!$this->isViewableImage()) {
       throw new Exception(
         'This file is not a viewable image.');
     }
 
     if (!function_exists('imagecreatefromstring')) {
       throw new Exception(
         'Cannot retrieve image information.');
     }
 
     $data = $this->loadFileData();
 
     $img = imagecreatefromstring($data);
     if ($img === false) {
       throw new Exception(
         'Error when decoding image.');
     }
 
     $this->metadata[self::METADATA_IMAGE_WIDTH] = imagesx($img);
     $this->metadata[self::METADATA_IMAGE_HEIGHT] = imagesy($img);
 
     if ($save) {
       $this->save();
     }
 
     return $this;
   }
 
   public function copyDimensions(PhabricatorFile $file) {
     $metadata = $file->getMetadata();
     $width = idx($metadata, self::METADATA_IMAGE_WIDTH);
     if ($width) {
       $this->metadata[self::METADATA_IMAGE_WIDTH] = $width;
     }
     $height = idx($metadata, self::METADATA_IMAGE_HEIGHT);
     if ($height) {
       $this->metadata[self::METADATA_IMAGE_HEIGHT] = $height;
     }
 
     return $this;
   }
 
 
   /**
    * Load (or build) the {@class:PhabricatorFile} objects for builtin file
    * resources. The builtin mechanism allows files shipped with Phabricator
    * to be treated like normal files so that APIs do not need to special case
    * things like default images or deleted files.
    *
    * Builtins are located in `resources/builtin/` and identified by their
    * name.
    *
    * @param  PhabricatorUser                Viewing user.
    * @param  list<string>                   List of builtin file names.
    * @return dict<string, PhabricatorFile>  Dictionary of named builtins.
    */
   public static function loadBuiltins(PhabricatorUser $user, array $names) {
     $specs = array();
     foreach ($names as $name) {
       $specs[] = array(
         'originalPHID' => PhabricatorPHIDConstants::PHID_VOID,
         'transform'    => 'builtin:'.$name,
       );
     }
 
     // NOTE: Anyone is allowed to access builtin files.
 
     $files = id(new PhabricatorFileQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withTransforms($specs)
       ->execute();
 
     $files = mpull($files, null, 'getName');
 
     $root = dirname(phutil_get_library_root('phabricator'));
     $root = $root.'/resources/builtin/';
 
     $build = array();
     foreach ($names as $name) {
       if (isset($files[$name])) {
         continue;
       }
 
       // This is just a sanity check to prevent loading arbitrary files.
       if (basename($name) != $name) {
         throw new Exception("Invalid builtin name '{$name}'!");
       }
 
       $path = $root.$name;
 
       if (!Filesystem::pathExists($path)) {
         throw new Exception("Builtin '{$path}' does not exist!");
       }
 
       $data = Filesystem::readFile($path);
       $params = array(
         'name' => $name,
         'ttl'  => time() + (60 * 60 * 24 * 7),
         'canCDN' => true,
         'builtin' => $name,
       );
 
       $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
         $file = PhabricatorFile::newFromFileData($data, $params);
         $xform = id(new PhabricatorTransformedFile())
           ->setOriginalPHID(PhabricatorPHIDConstants::PHID_VOID)
           ->setTransform('builtin:'.$name)
           ->setTransformedPHID($file->getPHID())
           ->save();
       unset($unguarded);
 
       $file->attachObjectPHIDs(array());
       $file->attachObjects(array());
 
       $files[$name] = $file;
     }
 
     return $files;
   }
 
 
   /**
    * Convenience wrapper for @{method:loadBuiltins}.
    *
    * @param PhabricatorUser   Viewing user.
    * @param string            Single builtin name to load.
    * @return PhabricatorFile  Corresponding builtin file.
    */
   public static function loadBuiltin(PhabricatorUser $user, $name) {
     return idx(self::loadBuiltins($user, array($name)), $name);
   }
 
   public function getObjects() {
     return $this->assertAttached($this->objects);
   }
 
   public function attachObjects(array $objects) {
     $this->objects = $objects;
     return $this;
   }
 
   public function getObjectPHIDs() {
     return $this->assertAttached($this->objectPHIDs);
   }
 
   public function attachObjectPHIDs(array $object_phids) {
     $this->objectPHIDs = $object_phids;
     return $this;
   }
 
   public function getOriginalFile() {
     return $this->assertAttached($this->originalFile);
   }
 
   public function attachOriginalFile(PhabricatorFile $file = null) {
     $this->originalFile = $file;
     return $this;
   }
 
   public function getImageHeight() {
     if (!$this->isViewableImage()) {
       return null;
     }
     return idx($this->metadata, self::METADATA_IMAGE_HEIGHT);
   }
 
   public function getImageWidth() {
     if (!$this->isViewableImage()) {
       return null;
     }
     return idx($this->metadata, self::METADATA_IMAGE_WIDTH);
   }
 
   public function getCanCDN() {
     if (!$this->isViewableImage()) {
       return false;
     }
 
     return idx($this->metadata, self::METADATA_CAN_CDN);
   }
 
   public function setCanCDN($can_cdn) {
     $this->metadata[self::METADATA_CAN_CDN] = $can_cdn ? 1 : 0;
     return $this;
   }
 
   public function isBuiltin() {
     return ($this->getBuiltinName() !== null);
   }
 
   public function getBuiltinName() {
     return idx($this->metadata, self::METADATA_BUILTIN);
   }
 
   public function setBuiltinName($name) {
     $this->metadata[self::METADATA_BUILTIN] = $name;
     return $this;
   }
 
   protected function generateOneTimeToken() {
     $key = Filesystem::readRandomCharacters(16);
 
     // Save the new secret.
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
       $token = id(new PhabricatorAuthTemporaryToken())
         ->setObjectPHID($this->getPHID())
         ->setTokenType(self::ONETIME_TEMPORARY_TOKEN_TYPE)
         ->setTokenExpires(time() + phutil_units('1 hour in seconds'))
         ->setTokenCode(PhabricatorHash::digest($key))
         ->save();
     unset($unguarded);
 
     return $key;
   }
 
   public function validateOneTimeToken($token_code) {
     $token = id(new PhabricatorAuthTemporaryTokenQuery())
       ->setViewer(PhabricatorUser::getOmnipotentUser())
       ->withObjectPHIDs(array($this->getPHID()))
       ->withTokenTypes(array(self::ONETIME_TEMPORARY_TOKEN_TYPE))
       ->withExpired(false)
       ->withTokenCodes(array(PhabricatorHash::digest($token_code)))
       ->executeOne();
 
     return $token;
   }
 
 
   /**
    * Write the policy edge between this file and some object.
    *
    * @param phid Object PHID to attach to.
    * @return this
    */
   public function attachToObject($phid) {
     $edge_type = PhabricatorObjectHasFileEdgeType::EDGECONST;
 
     id(new PhabricatorEdgeEditor())
       ->addEdge($phid, $edge_type, $this->getPHID())
       ->save();
 
     return $this;
   }
 
 
   /**
    * Remove the policy edge between this file and some object.
    *
    * @param phid Object PHID to detach from.
    * @return this
    */
   public function detachFromObject($phid) {
     $edge_type = PhabricatorObjectHasFileEdgeType::EDGECONST;
 
     id(new PhabricatorEdgeEditor())
       ->removeEdge($phid, $edge_type, $this->getPHID())
       ->save();
 
     return $this;
   }
 
 
   /**
    * Configure a newly created file object according to specified parameters.
    *
    * This method is called both when creating a file from fresh data, and
    * when creating a new file which reuses existing storage.
    *
    * @param map<string, wild>   Bag of parameters, see @{class:PhabricatorFile}
    *  for documentation.
    * @return this
    */
   private function readPropertiesFromParameters(array $params) {
     $file_name = idx($params, 'name');
     $file_name = self::normalizeFileName($file_name);
     $this->setName($file_name);
 
     $author_phid = idx($params, 'authorPHID');
     $this->setAuthorPHID($author_phid);
 
     $file_ttl = idx($params, 'ttl');
     $this->setTtl($file_ttl);
 
     $view_policy = idx($params, 'viewPolicy');
     if ($view_policy) {
       $this->setViewPolicy($params['viewPolicy']);
     }
 
     $is_explicit = (idx($params, 'isExplicitUpload') ? 1 : 0);
     $this->setIsExplicitUpload($is_explicit);
 
     $can_cdn = idx($params, 'canCDN');
     if ($can_cdn) {
       $this->setCanCDN(true);
     }
 
     $builtin = idx($params, 'builtin');
     if ($builtin) {
       $this->setBuiltinName($builtin);
     }
 
     $mime_type = idx($params, 'mime-type');
     if ($mime_type) {
       $this->setMimeType($mime_type);
     }
 
     return $this;
   }
 
   public function getRedirectResponse() {
     $uri = $this->getBestURI();
 
     // TODO: This is a bit iffy. Sometimes, getBestURI() returns a CDN URI
     // (if the file is a viewable image) and sometimes a local URI (if not).
     // For now, just detect which one we got and configure the response
     // appropriately. In the long run, if this endpoint is served from a CDN
     // domain, we can't issue a local redirect to an info URI (which is not
     // present on the CDN domain). We probably never actually issue local
     // redirects here anyway, since we only ever transform viewable images
     // right now.
 
     $is_external = strlen(id(new PhutilURI($uri))->getDomain());
 
     return id(new AphrontRedirectResponse())
       ->setIsExternal($is_external)
       ->setURI($uri);
   }
 
 
 /* -(  PhabricatorApplicationTransactionInterface  )------------------------- */
 
 
   public function getApplicationTransactionEditor() {
     return new PhabricatorFileEditor();
   }
 
   public function getApplicationTransactionObject() {
     return $this;
   }
 
   public function getApplicationTransactionTemplate() {
     return new PhabricatorFileTransaction();
   }
 
   public function willRenderTimeline(
     PhabricatorApplicationTransactionView $timeline,
     AphrontRequest $request) {
 
     return $timeline;
   }
 
 
 /* -(  PhabricatorPolicyInterface Implementation  )-------------------------- */
 
 
   public function getCapabilities() {
     return array(
       PhabricatorPolicyCapability::CAN_VIEW,
       PhabricatorPolicyCapability::CAN_EDIT,
     );
   }
 
   public function getPolicy($capability) {
     switch ($capability) {
       case PhabricatorPolicyCapability::CAN_VIEW:
         if ($this->isBuiltin()) {
           return PhabricatorPolicies::getMostOpenPolicy();
         }
         return $this->getViewPolicy();
       case PhabricatorPolicyCapability::CAN_EDIT:
         return PhabricatorPolicies::POLICY_NOONE;
     }
   }
 
   public function hasAutomaticCapability($capability, PhabricatorUser $viewer) {
     $viewer_phid = $viewer->getPHID();
     if ($viewer_phid) {
       if ($this->getAuthorPHID() == $viewer_phid) {
         return true;
       }
     }
 
     switch ($capability) {
       case PhabricatorPolicyCapability::CAN_VIEW:
         // If you can see the file this file is a transform of, you can see
         // this file.
         if ($this->getOriginalFile()) {
           return true;
         }
 
         // If you can see any object this file is attached to, you can see
         // the file.
         return (count($this->getObjects()) > 0);
     }
 
     return false;
   }
 
   public function describeAutomaticCapability($capability) {
     $out = array();
     $out[] = pht('The user who uploaded a file can always view and edit it.');
     switch ($capability) {
       case PhabricatorPolicyCapability::CAN_VIEW:
         $out[] = pht(
           'Files attached to objects are visible to users who can view '.
           'those objects.');
         $out[] = pht(
           'Thumbnails are visible only to users who can view the original '.
           'file.');
         break;
     }
 
     return $out;
   }
 
 
 /* -(  PhabricatorSubscribableInterface Implementation  )-------------------- */
 
 
   public function isAutomaticallySubscribed($phid) {
     return ($this->authorPHID == $phid);
   }
 
   public function shouldShowSubscribersProperty() {
     return true;
   }
 
   public function shouldAllowSubscription($phid) {
     return true;
   }
 
 
 /* -(  PhabricatorTokenReceiverInterface  )---------------------------------- */
 
 
   public function getUsersToNotifyOfTokenGiven() {
     return array(
       $this->getAuthorPHID(),
     );
   }
 
 
 /* -(  PhabricatorDestructibleInterface  )----------------------------------- */
 
 
   public function destroyObjectPermanently(
     PhabricatorDestructionEngine $engine) {
 
     $this->openTransaction();
       $this->delete();
     $this->saveTransaction();
   }
 
 }