How (or why) does 2584 "obsolete" 1867?
This may simply come down to my not understanding how to read RFCs, but from what I can discern, 1867 describes how file uploading works with HTML forms and 2584 is about a MIME type not used in HTML forms. Two completely different things?
RFC 2584 marks the end of the Internet Engineering Task Force's involvement in the HTML specification. Further work was continued by the World Wide Web Consortium.
RFC 2584 reads:
RFC 2584 obsoletes RFC 1867 and many other RFCs in the sense that HTML in its entirety was removed from the IETF's purview. This obsolesence of its previous HTML-related RFCs affirms that decision. And if you were to search for "HTML" on the IETF's RFC site, you'd see there's virtually no HTML-related activity since RFC 2584.
Also note that RFC 1867 was an experimental RFC. According to the IETF's "The Tao of IETF":