Unable to upload files to the wiki

I still can't upload presentations from FOSDEM to the wiki.
Whatever format I try (open document formats or pdf) I get a warning  
of the form
Upload warning

".pdf" is not a recommended image file format.

but it's not just a warning, it's actually an error ... because it  
doesn't upload the file.



I suppose we could put files on the ftp site and link to them, but  
that's clumsy (and most people don't have access to the ftp site).

How can we reconfigure the wiki to allow files to be uploaded?