Lucy, if another site is linking directly to the midi file from their page then Google Preview will load your file, even when previewing the other site's page. I have confirmed examples of this in my server logs.
So I have a choice - add the file type to robots.txt and end up with a lot of files indexed with the idiotic "we don't know what this is because it's blocked" description or live with the preview problem. I considered giving Google a 503 but then they start to doubt the quality of my site so... lose, lose, lose.
I've decided to return 503 anyway, it's best for my site and Google can work it out on their end, or not.