Well, you can and you can't. See the problem is that you have no idea what the next request will be about! So if you're sending the same kind of request from the client you can respond with a payload in the mime type that is being expected. But for your usecase you could receive a request for /style.css and respond with /favicon.ico if a client decided to make the requests in an order that you did not anticipate.

If you get lucky it will work, but if you're unlucky then you'll be sending out the wrong payloads on all but the first request.

The only reason this trick worked for the webcam is because it knows ahead of time what kind of request will come (the request for the next frame). That's why it can anticipate.