So about a week ago, there was a thread about tiff files not working as well as they should. Here's a status update:
The good news:
The 16-bit greyscale tiff files not rendering bug (T116947) is now fixed and the fix is live on commons. This affected a lot of images from the library of congress. Previously they showed up as either all black, or mostly white with a couple black lines. Now they work.
The more meh news:
We've identified a fix for the issue where pages after the first page of a large tiff do not render (T117349). This is a good step forward, but the fix is currently stuck pending review and I'm not sure what time frame the fix will be reviewed in.
Tiff (and djvu) files on the beta cluster do not render (T116816). The people who maintain the beta cluster have put it on their list of next things to do. So hopefully it will be fixed soon. In the mean time, if you really need to see what a tiff would look like uploaded but don't want to put it in real commons, you can use test.wikipedia.org (but alas, gwtoolset does not work in that domain)
-- -bawolff
p.s. Fun fact that might interest the Basel University Library folks - If you order all the tiffs on commons by file size, 18 of the top 20 are from Basel University Library.