Fix a stray comma on File previews

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

{F248962}

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