R1:5493f028dca6

Provide a simple "Attach File" explicit workflow for files referenced but not attached

Summary: Ref T13682. Allow users to manually attach files which are referenced (but not attached) via the UI.

Test Plan: Reference files via `{F...}`, then attached them via the UI workflow.

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21837