We use xml as a file extention for Uppaal models. This does not contribute any information about what is inside the file other than it is xml (it can be almost anything).
This has a consequence that the files cannot be associated with applications in OS shell (Windows Explorer, Nautilus, ...), and the user has to visit two locations (start Uppaal and then point it to the Uppaal model) instead of just shell taking care of it. This is especially true for the GUI-driven work conditions.
So I suggest to change the file name extension to something more meaningful.
I suggest UTA (Uppaal Timed Automata) or USM (Uppaal System Model).
It can also be just uppaal, I am open to suggestions :-)