rebhana:

You have to add non-existant types to /usr/share/applications/defaults.list manually.

And yes, I am aware that the GUI, not written by me, could use some serious work. I would like to try making a new GUI in qt as part of my self-training. But I don't know if or when I'll get the time to do that.

So probably the best thing for now is to start documenting all the quirks and tricks of this GUI, and if I get an opportunity, I'll write a new GUI.