Fix lightbox downloads for embeded images and a warning

Summary:
I refactored this recently and accidentally dropped the download URI.

Also fix a warning with, e.g., files named `README`.

Test Plan: Clicked a thumb, clicked "Download", got a file.

Reviewers: chad, btrahan, dctrwatson

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7341