Fix external redirect flagging issue with image thumbnails

Summary: Fixes T5894. This needs some improvement when we lay in real CDN stuff, but should get all the cases right for now.

Test Plan: Thumbnails work properly again.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5894

Differential Revision: https://secure.phabricator.com/D10299