application/x-dia-diagram is a subclass of text/xml, but isn't marked as such.
Created attachment 7955 [details] [review] Patch to correct the bug
2007-02-02 Bastien Nocera <hadess@hadess.net> * freedesktop.org.xml.in: Make Dia's format a sub-class of text/xml as reported by Tom Parker <freedesktop@tevp.net> (Closes: #9241)
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.