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
Oh, it's not another site, it's my own: the midi belongs to the page being previewed. Twice, at that: as an embedded file and then again in the <noscript> version.
I just thought it was ridiculous for Preview to include something that cannot possibly affect the look of the page in any way whatsoever. That's why I threw in the rhetorical question about pages that happen to contain multi-megabyte movies.