It would be nice if there were a quick way to add a file extension so that it automatically started using a particular existing grammar, AND hooked up the extension in the OS, so when you typed a filename with that extension it automatically invoked Atom.
Unless I’m mistaken Atom doesn’t do anything to affect OS behavior. It would obviously be OS dependent which is something that is avoided when possible.
I’m pretty sure that Atom associates some file extensions on initial installation. I don’t have a Windows machine or VM to double check that at the moment though.