If you follow a few links from that IANA (MIME) Media Types page, you'll find RFC 4288, which describes the process. There are at least four "trees":
Your question is about the standards tree:
Registrations in the standards tree MUST be approved by the IESG and MUST correspond to a formal publication by a recognized standards body.
- A "recognized standards body" drafts a proposal
- It is also announced on an official mailing list
- Interested parties discuss it
- The IESG approves it (or not)
- An appointed "media types reviewer" makes sure the proposal meets all detail requirements
- It is published by IANA
So to answer your question, a standards body must first decide it is "popular enough" to pursue, and then the Internet Engineering Steering Group (a part of the IETF, Internet Engineering Task Force) must agree.