R1:9181929ebc2f

Give files uploaded to objects a very restrictive view policy

Summary:
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…