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