Fix broken file PHID extraction that causes Pholio uploads to crash
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.
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
Maniphest Tasks: T15105
Differential Revision: https://we.phorge.it/D25058