Viewerize ArcBundle file loading callbacks

Summary: Ref T603. Clean these up and move them to a single place.

Test Plan:
  - Downloaded a raw diff.
  - Enabled "attach diffs", created a revision, got an email with a diff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7179