Even though the registered MIME type for MIDI files is audio/midi, some browsers are not set up to recognize it as such; instead, they look for audio/x-midi. There are two things you can do to address this:

1. Configure your browser to treat documents of type audio/midi correctly. This is the type that Apache sends by default. This may not be workable, however, if you have many client installations to change, or if some or many of the clients are not under your control.
2. Instruct Apache to send a different Content-type header for these files by adding the following line to your server's configuration files:

   AddType audio/x-midi .mid .midi .kar

   Note that this may break browsers that do recognize the audio/midi MIME type unless they're prepared to also handle audio/x-midi the same way.