The Internet media type text/markdown is in the process of being registered.
I am the author of draft-ietf-appsawg-text-markdown: The text/markdown Media Type, which has been adopted as an IETF Applications Area Working Group item. It’s making its way through. Comments on the draft should be directed to the apps-discuss mailing list.
Regarding the file extension: my draft registers both .md and .markdown. Media type registrations are not authoritative on the topic of file extensions; however, nothing is. Personally I prefer .markdown. I know, however, there are enough README.md (and other) files out there in existence, that we should not pretend about .md these days.