@elextr Thank you. That worked, but the purpose of my bug report remains; I hope Geany will add `.mjs` as a default extension, so other JS developers don't have to endure any inconvenience regarding `.mjs` files.
Additionally, when a user changes a Filetype, for an extension Geany doesn't recognize via `Document > Set Filetype`, a very nice automation would be for Geany to prompt the user: "Would you like Geany to always open .mjs files as the JavaScript filetype?"
Lastly, on the information bar, at the bottom of Geany, I find it very un-intitive that you cannot just **click** `filetype: none` as a shortcut for achieving the same thing that `Document > Set Filetype` achieves.
These enhancements are certainly not a necessity, but I know they'll be appreciated by newcomers.