Comment # 2 on bug 1170243 from Andreas Stieger

Featherpad already identifies itself as supporting text/plain:

/usr/share/applications/fetherpad.desktop
> MimeType=text/plain;

https://github.com/tsujan/FeatherPad/blob/master/featherpad/data/featherpad.desktop#L29

An application does not define itself to be the *default* via it's own .desktop
file. That is up to the desktop environment, and the user's preferences.