I direct your attention to RFC1521, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies".

Though the title says "Mail", that limitation is an anachronism. MIME types are also used to specify the content type of an HTTP response (e.g., whether the response is HTML or an image type).

And interestingly enough, the first hit you get if you type "mime" at google is a copy of RFC1521. (If you try "mime type", the RFC is at the bottom of the page.) Google is nifty. Try it sometime.