From MediaWiki-General-or-Unknown in January:
<GerardM-> Brion, why do we not support tiff files ?
<GerardM-> this would make a big difference for the image restoration projects
<brion> GerardM-: i think we haven't quite got round to it. tiffs are a bit complicated in that it's a meta-format with many possible actual encodings
<brion> check if there's an extension for it already in the works and plop it on my queue to look at :)
<GerardM-> it prevents the restorations to be taken seriously by people outside or WMF ... you can not repeat the work from compressed files
<GerardM-> Brion, at the start of the fundraiser, localisation software was written for the banner ..
<brion> yes, tiffs are great. :) i'm convinced -- just make sure that any existing extension or bug gets bumped and add me as CC
I can't find a bug filed regarding TIFF support, so I've filed this one and CC'd Brion.