* (bug 31282) Fix use of ForeignAPIRepo/InstantCommons TIFF images when $wgTiffThumbn...

* (bug 31282) Fix use of ForeignAPIRepo/InstantCommons TIFF images when $wgTiffThumbnailType is left unset.

When pulling files over ForeignAPIRepo, the source site is what creates the thumbnails; we don't care whether we can locally produce thumbs of TIFFs.
This feels a bit hacky, and may indicate we need to better refactor media handlers & file repositories. As long as the source site can produce a thumbnail image URL or a chunk of HTML to export, the local site shouldn't have to touch any of that stuff and should just be passing data through.