Atari at least STE and ST and up can read 720 Kb formatted PC disks, Falcon can read 1.44 Mb formatted PC disks. Macintosh can also read 720 Kb and 1.44 Mb formatted PC disks. So it wont any problem for you to use *.mid files in your Atari. Just format an DD 702 Kb floppy disk in an PC then save all files on it, then can you read the disk in your Atari.