Fixes T4589. This implements much better policy behavior for files that aligns with user expectations.
Currently, all files have permissive visibility.
The new behavior is:
- Files uploaded via drag-and-drop to the home page or file upload page get permissive visibility, for ease of quickly sharing things like screenshots.
- Files uploaded via the manual file upload control get permissive visibility by default, but the user can select the policy they want at upload time in an explicit/obvious way.
- Files uploaded via drag-and-drop anywhere else (e.g., comments or Pholio) get restricted visibility (only the uploader).
- When the user applies a transaction to the object which uses the file, we attach the file to the object and punch a hole through the policies: if you can see the object, you can see the file.
- This rule requires things to use ApplicationTransactions, which is why this took so long to fix.
- The "attach stuff to the object" code has been in place for a long time and works correctly.
I'll land D8498 after this lands, too.