Remove TYPE_FILES from Conpherence

Summary: I believe these are left over from widgets, when we added a "Files" widget that kept track of everything added to the window.

Test Plan: Added files to a Conpherece, Set an image when editing. Anything else?

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16567