Ubuntu 18:04: How do I add an application to the list of default applications?

I want to install a default handler for MIDI files (such as those embedded in a Wikipedia page such as this: https://en.wikipedia.org/wiki/Ninth_chord). I tried installing two (timidity and Yoshimi). Both are able to play MIDI files from the command line or the application’s menu. However, neither of these appear in the list of default applications.…

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