Fix file PHID extraction in Pholio

Summary: Ref T13682. After some change prior change likely connected to T13682, file PHIDs fail to extract properly in Pholio.

Test Plan:

• Created a new Pholio mock with at least one image.
• Before: fatal when extracting data from this datastructure.
• After: clean mock creation.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21858