.tif thumbnails from a foreign repo (Wikimedia Commons) don't work on a local wiki, unless $wgTiffThumbnailType is specified in LocalSettings.php:
$wgTiffThumbnailType = array( 'jpg', 'image/jpeg' );
These thumbnails should just work, regardless if $wgTiffThumbnailType is set on the local wiki.
Version: 1.20.x
Severity: normal
Fixed in r98812, marking for 1.18 etc.
This feels a bit hacky -- TiffHandler::canRender() right now checks the local tiff thumbnailing settings to decide if we can render thumbs or not, but for files loaded over ForeignAPIRepo we generally are using premade thumbnails from somewhere else and it doesn't matter if we could create them locally.
May wish to do further refactorings on the file, file repo, and media handler APIs to better distinguish between things that need to be done locally and things that don't.
Note that there are other things like multipage support that are also broken in this configuration...
Starting to collect some notes for further work at [[mw:Requests for comment/Refactor on File-FileRepo-MediaHandler]].