R1:eb9dcd6fbaf5

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…