The most appropriate existing content-type to describe kernel images is application/octet-stream. It basically means, "unidentified binary content - do not attempt to interpret".

You could invent a type like application/x-empeg-kernel (the "x-" is important), and anything that doesn't understand that should treat it the same as application/octet-stream. That would enable somebody to install a "player" for that type that uploaded it direct to the player. But that's probably OTT.

(Except to MS Internet Explorer of course, which seems to ignore content types in favour of "guess based on contents". But there's nothing you can do about that, and I doubt its heuristics would get it wrong in this case. It is a nuisance for those with HTML tutorials that have links to view the source as text/plain - and the browser "corrects" it to text/html. )