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 after we extracted the PHID. So we'd extract the entire text of the paste as a "file PHID", which wouldn't work.
Instead, extract file PHIDs later. This impacts a couple of other applications (Conpherence, Pholio) which receive an object or have an unusual file-oriented transaction.