Consolidate handling of special properties for newly uploaded files

Summary:
Fixes T5849. When a new file is created, we might have to actually write the data to a storage engine, or we might be able to just point at data which is already there.

Currently, these two paths handle `$params` with different code and mild behavioral differences. Instead, have them call the same code so they get the same behavior.

Test Plan:
  - Uploaded the same file multiple times to home page.
  - Uploaded the same file multiple times as profile picture.
  - Generated files via Diffusion.
  - All the files got the expected properties, whether they were reusing data or not.

Reviewers: btrahan, 20after4

Reviewed By: 20after4

Subscribers: epriestley

Maniphest Tasks: T5849

Differential Revision: https://secure.phabricator.com/D10216