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.

Test Plan:

• Created and modified a paste.
• Confirmed both pathways do old and new storage writes.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21817