Please enable the PagedTiffHandler extension on all wikis, to allow display of TIFF files. Documentation of the extension can be found at http://www.mediawiki.org/wiki/Extension:PagedTiffHandler.
There is broad support for this on commons, at least: http://commons.wikimedia.org/w/index.php?title=Commons:Village_pump&oldid=37854914#Support_for_TIFF_files_-_just_ask_for_it.21.
Wikimedia Deutschland contracted Hallo Welt to develop this extension. I have reviewed the code and found it sane enough. A first look by some of the core devs revealed the need for some cleanup, but no fundamental problems so far. The code is available at http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/PagedTiffHandler/.
Please review the extension, let us know what still needs to be fixed, and then disable it, so we can enjoy tiff files directly. Thanks!