What is your targeted OS?
If windows
that information is stored in the regristry and you can set it using Godot's OS.execute()
with a VB or PoweShell script. A neat trick would be to do the file association on your PC then in the registry editor do a search for your game name then export the .reg
file which you can later use for the distribution of your game by resetting it.
Have not used windows since v7 but it used to be somewhere inHKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
and may have changed in recent years