Make an appimage executable is absolutely easy. But less easy is to associate that application with its mime-type files. Especially if that type of file doesn’t exist already on your system.
A very easy way, then, is to go to the appimage folder and, in a terminal, do
(sudo) ./appimage-name installWith this command all will be ready, correctly set, as if that application wouldn’t be an appimage, but a “regular” one.