That's tangentially related to the Vary header. Not only Accept can go into its value, you know.
And an optimal solution IMHO would be for the origin server to simply return 302 to a specific resource, selected upon the value of the Accept header:
GET /thumb.php?id=kekw HTTP/1.1
Accept: image/avif,image/webp,image/apng,image/svg+xml,image/*,*/*;q=0.8
HTTP/1.1 302 Found
Location: /media/thumb.jpg?id=kekw
Vary: Accept
GET /media/thumb.jpg HTTP/1.1
Content-Type: image/jpegSure, except I doubt most people want to uglify all their urls with extensions for occasional alternates. Plus, if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.
I had thought about recommending that people just use an alternate link as intended, to point to an alternate format. I think that would work best using existing web standards as intended, but it has the downside of initially serving the original format regardless of the content type.
> if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.
Why? It has no "Vary" header, and it's the one that's supposed to get cached anyhow.
An optimal solution would involve: the response listing which alternate content-types can be returned for that endpoint, the cache considering the accept header, if it sees a type from the alternates list higher in the accept header priority than whatever it has in cache, then it would forward the request to the server. Once it had all the alternatives in cache, it would pass them through according to the accept without hitting the server.
The closest existing header to the above would be the link header, if you give it rel=alternate, and type as the mime type. It's not clear what href you would be, since it usually is to a different document, but we want the same url but a different mime type. So clearly this would be an abuse of the header, but could work.