This LilyPond output shows UTF-8 characters incorrectly in Firefox 57.0.4 because there’s no encoding specified in the content-type HTML header (it says just text/plain—LilyPond files have no own MIME type?).
No, LilyPond files don't have an official MIME type. On their discussion forums, text/x-lilypond has been brought up. I'm testing on Vagrant, and now it doesn't work on Chrome! (Safari unaffected). In progress.
Content-Type: text/plain; charset=utf-8 would also be better than now, so that the browser wouldn’t use some one-byte encoding instead of UTF-8. If LilyPond once gets an official MIME type, it can be changed then.
Would there be a preferable name? There is not necessarily any descriptive naming information in the source or the <score tag. At least with the hash the related files can be easily paired.
I don’t think we can easily find a more descriptive file name than the current one. Maybe a new attribute can be introduced for the <score> tag, but that, of course, won’t be available for the currently existing scores.
This is odd; the download attribute does not seem to have an effect (in either Chrome or Safari).
On another note, I do not think it is possible to set the MIME type or any headers through the link.