R1:7b290b94a726

Correct file PHID extraction on image update in Pholio

Summary: Ref T12776. This extraction of file PHIDs extracted "PholioImage" object PHIDs (`PHID-PIMG-...`), not "File" PHIDs (`PHID-FILE-...`). Instead, dig into the Pholio images and actually extract the file PHIDs. This method is now similar to the `PholioImageFileTransaction` method, which works already.

Test Plan:
- Create a mock.
- Update one of the images.
- In Files, view the "Attached" tab of the updated image.
- Before patch: not…