There's a few ways to add a mime type, and using $overrides is the difficult way. The only reason you'd use that is if you were writing a plugin or something that allowed the user to upload files and you were actually calling wp_handle_upload() yourself.
The easiest way to add a mime type for everything else is to make use of the "upload_mimes" filter hook. You can do this in a plugin or in functions.php for your theme or whatever.
Make a function like this:
Then add the filter:
This will cause your function to get called and passed the $mimes array. Then you can just add your mime type to the array and return that array.