Thanks. Clearly, that menu hasn't been touched in a long time....
I've added this to our issue tracker.
For the time being, Ironfist follows the PoL map format. Many Ironfist maps will load just fine with the PoL editor. So, if you put kobolds in a castle's garrison, the map will open, but it will crash if you try to edit the castle. This fact became really useful last night when I found a bug in the Ironfist editor shortly before release, and wound up working with it in the normal editor....
I generally don't think sorting by extension is a good way to categorize maps on the Internet, so I see this as one of the smaller problems. At some point, we will necessarily need to switch to a new map format, and then the extension will change accordingly.