Create a New Mimetype
We have to teach the system about GuitarPro as something called a "mimetype". To do that, we need to create a mimetype package file that contains info about how to identify GuitarPro files.
Create a new file called "guitarpro.xml" at "/usr/share/mime/packages"
In this file we need to make some rules about how to match files to this ...
Compare the .desktop file of mtpaint with another .desktop file of an app that works and see what's missing or what's different.
You might need to add %U at the line Exec, so you will have: Exec=mtpaint %U