[Mirrors] Wrong MIME type from some mirrors

[David Boswell]

> I've found some mirrors returning the wrong MIME type for XPIs:
> 
> http://ftp.cc.uoc.gr/mirrors/mozdev/
> http://mirrors.wikifusion.info/mozdev/

This should now be fixed.  We couldn't get in touch with the admins for
either of those servers, so we pulled them from the rotation.  We can
add them back once the MIME types are fixed.  If anyone notices any
other problems with how XPIs are being served, please let us know.

David