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