Select multiple files with different extensions > hold [Control] and right click > Get Summary Info > Open with (select all) > click [Change All...]. The files

Select multiple files with different extensions > hold [Control] and right click > Get Summary Info > Open with (select all) > click [Change All...]. The files don't need to be valid, just have the extension you want to associate, so you can do something like `touch _.mp3 _.mp4 _.m4a _.mkv` then select those files and set them all at once.