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Image/Audio `format=` emits image/jpg, audio/mp3, audio/m4a — unregistered MIME types that disagree with the `path=` suffix table

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
84/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
api

Research direction

Start in src/mcp/server/fastmcp/utilities/types.py and compare the existing path suffix table with the format= handling for Image and Audio; docs/servers/media.md explains the data= usage. Run the relevant Image and Audio MIME-type tests, and consider the issue's parametrized cases; done means registered aliases match path-based results while unknown formats retain their fallback.

Written by the indexing model from the issue text.

Description

v1 v2
Initial Checks
  • I confirm that I'm using the newest release of my release line (2.2.0)
  • I have searched GitHub issues and found no similar issue
Release Line

2.x (current stable). The same code exists on v1.x (src/mcp/server/fastmcp/utilities/types.py).

Description

Image and Audio compute the MIME type two different ways. With path= they use a suffix table (.jpg -> image/jpeg, .mp3 -> audio/mpeg, .m4a -> audio/mp4). With format=, which docs/servers/media.md says to pass whenever you use data=, they just do f"image/{format}" / f"audio/{format}". So the same name gives different results depending on which keyword it arrives through, and format= puts non-registered types on the wire:

  • Image(data=..., format="jpg") -> mimeType: "image/jpg" (should be image/jpeg)
  • Audio(data=..., format="mp3") -> audio/mp3 (should be audio/mpeg)
  • Audio(data=..., format="m4a") -> audio/m4a (should be audio/mp4)

This is easy to hit: switching a tool from Image(path="chart.jpg") to Image(data=buf.getvalue(), format="jpg") silently changes the wire output from image/jpeg to image/jpg, and a consumer that validates image MIME types rejects it.

Expected: format="jpg" and path="x.jpg" give the same registered type.

Proposed fix: route format= through the same table the path= branch already has, keeping image/<format> / audio/<format> as the fallback for names outside it (so format="bmp" etc. are unchanged). I have this fix with parametrized tests ready locally (100% branch coverage on types.py, ruff/pyright clean) and would like to open the PR if this is accepted.

Example Code
from pathlib import Path
from mcp.server.mcpserver import Audio, Image

print(Image(data=b"\x00", format="jpg").to_image_content().mime_type)   # image/jpg   (expected image/jpeg)
print(Audio(data=b"\x00", format="mp3").to_audio_content().mime_type)   # audio/mp3   (expected audio/mpeg)
print(Audio(data=b"\x00", format="m4a").to_audio_content().mime_type)   # audio/m4a   (expected audio/mp4)

# The same helpers get it right when the name arrives as a file suffix:
print(Image(path=Path("x.jpg"))._mime_type)   # image/jpeg
Python & MCP Python SDK
Python 3.12.13
mcp 2.2.0 (also reproduced on main @ f1b6589)

Disclosure: found, reproduced and the draft fix tested locally with the help of an AI coding agent (Claude Code); I reviewed the report before filing it.

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