Fix broken file PHID extraction that causes Pholio uploads to crash

Summary:
A commit earlier this year modified the structure of the file upload transaction data value, by nesting the array of file upload PHIDs in another array.
The extractFilePHIDs method was not updated to deal with that change though, therefore new mock uploads via Pholio would crash.
This patch fixes that method so it can process the updated transaction data.

Resolves T15105

Test Plan: Patched my live Phabricator installation with this fix and successfully uploaded new Pholio mockups.

Reviewers: O1 Blessed Committers, Cigaryno, Matthew

Reviewed By: O1 Blessed Committers, Cigaryno, Matthew

Subscribers: Matthew, speck, tobiaswiese, valerio.bozzolan, Cigaryno

Tags: Pholio

Maniphest Tasks: T15105

Differential Revision: https://we.phorge.it/D25058