R1:06de60599288

Extract PHIDs from transactions later, fixing Paste extraction/attachment

Summary:
Fixes T9787. Currently, file PHID extraction logic happens very early, before we normalize/merge/etc the transactions.

In D14390, I changed how the CONTENT transaction works: before, callers would pass in a file PHID. Afterward, they just pass in the content.

Passing in the content is generaly easier and feels more correct, but inadvertenly broke PHID extraction because converting the content into a file PHID now happened…