We're tightening up security on the uploads due to exploitable Internet Explorer and Safari bugs that allow for scripting attacks to steal session and saved password cookies.
For the moment uploads are limited to the default extension whitelist: 'png', 'gif', 'jpg', 'jpeg', 'ogg'. Other types of files can't be uploaded to the wikis until we expand it again, but files already uploaded remain accessible.
-- brion vibber (brion @ pobox.com)