Uploading file type error

Since the update to 7.0.3, the Media Uploader addon does not allow any user to upload a file with the m4a extension. Upon attempting to do so, I get the default "Not allowed file type" message.

I tried putting the following code in the current active theme functions.php file, but it still doensn't work

add_filter("upload_mimes", function ($t, $u) {
   if (empty($t["m4a"])) {
   $t["m4a"] = "audio/x-m4a";
}
   return $t;
}, 10, 2);