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 attached to mock.
• After patch: properly attached to mock.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12776

Differential Revision: https://secure.phabricator.com/D18042