I'm using a concrete example to explain an abstraction. I'm not suggesting to hard code `.mjs` specifically.
I'm suggesting that any time a Geany user encounters "Filetype: None", that after they set that file type using `Document > Set Filetype` (for example), that Geany takes note of what ever that extension is, and asks the user "Would you always like to associate files with the extension ".???" with "the format you've just specified".