R1:a8a38835e15e

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…