I just went and checked. By default extensions are hidden and can't be changed. If you hit the menu thing and select "Show all extensions" you can now edit file

I just went and checked. By default extensions are hidden and can't be changed. If you hit the menu thing and select "Show all extensions" you can now edit file extensions. Seems appropriate to me. Changing a files extension is pretty unusual and will break things in ways users don't understand. But if you do know what you are doing, it took me 20 seconds to work out how to do it without reading a manual.