Don't do image stuff with unviewable images

Summary: If a file isn't a viewable image, don't try to figure out metadata (size, etc.) when rendering a `{F...}` tag in Remarkup.

Test Plan: Uploaded a .rtf, added it as `{F1}` in a new Maniphest task, saw no errors in the dark console.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2479

Differential Revision: https://secure.phabricator.com/D4837