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 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.

Test Plan:

• Made a new paste, verified the raw file attached to it properly.
• Made and updated a mock, verified all the files attached properly.
• Updated a Conpherence room image, verified the files attached properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9787

Differential Revision: https://secure.phabricator.com/D14494