One of the things that mime types are used for is to specify the default program to associate with a file extension. Unfortunately, this is currently broken with Gnome Commander (at at v18.104.22.168). To fix it, you’ll need to add a line to the following file:
Create that file if it doesn’t exist and add [Default Applications] on the first line. Then add the mime type (as shown on the gnome commander error dialog when you try to open your file) and specify the program to use. For example, for PNG file extension you might want to use Eye of Gnome (eog) so you’d add:
Save the file and you’re done. If it still doesn’t work, you should check this thread on the Ubuntu forums. Tested with Zorin OS 9. Hopefully this is fixed in a future version.