It is impossible to upload large file via upload form in https://secure.phabricator.com/file/upload/, whenever you try uploading large file, eg.: 20mb.
It will crash with:
No configured storage engine can store this file. See "Configuring File Storage" in the documentation for information on configuring storage engines.
Uploading via drag&drop - works as expected, file is chunked and uploaded, arc upload works also.