Add mime type proposal document

[jakemac53]

Motivation and Context

Related to #54, this proposal came out of some discord discussions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[olaservo]

Hi @jakemac53 , thanks again for opening this, and for following up in Discord and the office hours where we talked through loosening the skill:// requirement.

I think the main concern that a hard skill:// MUST would exclude servers with domain-native URI schemes (package-root:, github://, etc.) has now been addressed in the draft Skills Extension SEP (#69). The SEP explicitly makes the scheme a SHOULD rather than a MUST.

The main difference that still exists between this proposal and the SEP is the identification mechanism: text/agent-skill MIME type vs. entry in skill://index.json. The SEP went with the index route because it aligns with the Agent Skills well-known URI index, so a host that already consumes .well-known/agent-skills/index.json over HTTP can consume skill://index.json over MCP with the same code. Adding text/agent-skill on top would give hosts two places to look for "is this a skill?" might fragment the detection path.

Given all that, I'm inclined to close this PR as superseded by #69 . A few options if we'd prefer to keep the exploration visible:

1. Close and reference from the SEP since the rationale is preserved in the git history and linked from the decision log.
2. Merge as a comparison doc in docs/ with a Status: Superseded by SEP draft #69 header.
3. Keep open if you think there's a case for dual identification (URI scheme and MIME type) that we should still debate.

Let me know which you'd prefer.

[jakemac53]

Yes, I will go ahead and close this. We discussed in the meeting as well that the file format isn't specific to skills either, which I did not realize (I just hadn't ever seen frontmatter in markdown previously). Given that, the correct mime-type for these is probably just text/markdown.