Fix a stray comma on File previews

Summary:
There's a comma to the lower-left of my profile picture here:

This is on a page like https://secure.phabricator.com/F248948

What's happening is that some render() method is returning a valid result like array($stuff, null). This is getting passed to JS as an array, which is implicitly join()'ing it into a string, adding a comma.

Instead, make sure we render these to strings on the server side before shipping them to the client.

Test Plan: No more comma on file previews.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10974