How do I create my spec file so that I my gui application can be launched by double clicking a filetype it is associated with via mimetype?

This is what I've done so far:

- I have updated my .desktop file so that it includes MimeType=application/x-supes-saved;

- In my spec I have successfully used xdg-mime and xdg-icon-resource so that files with my custom extension (.supes) has my custom icon.

But when I double click on one of my files, I am prompted with a modal dialog "There is no application installed for Superman Saved Game files".  When I opt to "Select Application" my application is not listed either.  (The application's desktop icon is successfully installed and runs under the application menu).

Any suggestions?  Thank you.