Fix displaying non-image files.
Summary: The Files application currently tries to render all browser-viewable files in an img tag, not taking into account if they're an image or not.
Test Plan: Looked at various image and non-image files.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Differential Revision: https://secure.phabricator.com/D4511