This is an ongoing headscratcher. If someone has a simple explanation I'd like to hear it.
Q: What do search engines want with .midi (sound) files?
I looked it up. The extension isn't used for anything else. There is no Sound Search. ("Google Play" is something else. What exactly I'm not sure: their answer to iTunes?) ".midi" is not one of the options on the File Type dropdown in Advanced Search.
You can use the word "midi" in a search string-- I've seen it-- but the search engine wouldn't need to GET the file. A simple HEAD is enough to confirm that it exists.
I guess I should be glad the robotic brain understands that it doesn't need to include a .dmg, .zip or .sit in a preview. Those are sized in megabytes. A .midi is just a few bytes.
Which of those isn't a music file? I couldn't find any. Admittedly I didn't go through the full 213,000 hits.
Or did you mean to point out that "filetype" is a search option, albeit one of the "you don't know unless you know" group? Interesting that the named files in the search results (second row, green on my browser) are .midi but the clickable links (top row) go to the page. No audio equivalent of hotlinking-- at least not yet.
Incidentally I just met another one: Preview downloads .ttf (font) files even though it doesn't use them. I have them in one directory's headings, so you can tell even at micro-preview-scale.
It's not a recording as such, no. You could think of it as a highly specialized synthesizer. That's why midi files are measured in kilobytes rather than megabytes.
The .midi format is unsurpassed for packing a lot of information into a tiny amount of space: f'rinstance "Three Blind Mice" as a three-part round weighs in at 1.6K, while all the music accompanying a ten-page article totals 30K. That's only the melody of course-- in instrument of your choice --not the singing. Closest you can get is "Choir oohs" or possibly "Choir ahhs". (All my midis originated with lilypond, which is the musical equivalent of a command-line interface.)
It was a nasty shock to discover that the iPad's Safari doesn't play midis, whether embedded or as freestanding files. I thought all browsers did.
The question remains: What do Search and Preview do with them? Sound files they can't play, fonts they don't use...