What is a media type (AKA MIME) Template?

Per iana.org, all media types except 17 of them have a template and those without a template are gif, jpeg, mpeg, QCELP, ief, external-body, partial, rfc822, mesh, vrml, alternative, digest, mixed, parallel, enriched, plain, and richtext . What is the template? Why don't the others have one?

The template itself is inside RFC 6838 - §5.6. It's just a plaintext document with placeholders that the registrant (someone or an org wanting a MIME registration) fills out and then submits to the IANA.

The "Template" column in the IANA.org Media Types Page links to the originally submitted forms for that registration.

I agree that using the term "template" in this context feels like a misnomer as they actually link to filled-out forms, rather than blank templates.

As for why some formats don't have a template, I suspect it's because of 3 main possibilities:

1. The format, and its usage on the Internet, predates the IANA registration system, and I speculate that the IANA added registrations for common media types encountered at the time. This is likely the case for GIF and JPEG which both date back to the 1980s.
2. The format was registered after RFC 6838 was published, which seems to deprecate the practice of submitting a template-form.
3. Some formats are part of the IANA and RFC systems themselves - and submitting a template form to oneself is just silly.

• The Media Types page references RFC 6838 (right at the top) and it contains instructions and procedures for new registrations:

Procedures for registering Media Types can be found in [RFC6838], [RFC4289], and [RFC6657].

• Originally the registration procedure was in RFC2048 which mentions using the template (emphasis mine):

2.3 - Registration Procedure

The following procedure has been implemented by the IANA for review and approval of new media types. [...] For registrations in the vendor or personal tree, the initial review step described below may be omitted and the type registered directly by submitting the template and an explanation( directly to IANA (at iana@iana.org).

• Previously RFC4288 (which replaced RFC2048) prescribes the template:

4.10 - Publication Requirements

[...] Such specifications MUST contain an appropriate media type registration template taken from Section 10. [...]

• Curiously, RFC6838 (which replaces RFC4288) doesn't describe using the template anymore, but the template is still included in the RFC. I'll admit I haven't read RFC6838 in detail enough to be able tell you exactly what the new procedure is, however.