Adjust paste content transaction to support earlier file PHID extraction
Summary: Ref T13603. New transactional file attachment extracts PHIDs earlier than the older mechanism did. Allow either pathway to save content and extract a PHID.
- Created and modified a paste.
- Confirmed both pathways do old and new storage writes.
Maniphest Tasks: T13603
Differential Revision: https://secure.phabricator.com/D21817