What is the MIME type for Markdown?

There is no official standard type, but text/markdown seems to be the most common de facto type. Most browsers and other reasonably sophisticated clients will likely see the text/ part and default to text/plain anyway, so there's not much difference.

One caveat, though: all types under the text/ hiearchy default to ISO-8859-1 for their character type in the relevant RFC standards. Most of the world has since moved on to UTF-8. So unless you're positive you won't be using any funny characters (or live in an old Windows world) you might want to specify it as follows:

text/markdown; charset=UTF-8