Image/Audio `format=` emits image/jpg, audio/mp3, audio/m4a — unregistered MIME types that disagree with the `path=` suffix table
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评估
调研方向
从 src/mcp/server/fastmcp/utilities/types.py 开始，将现有的路径后缀表与 Image 和 Audio 对 format= 的处理进行比较；docs/servers/media.md 说明了 data= 的用法。运行相关的 Image 和 Audio MIME 类型测试，并考虑 issue 的参数化用例；完成的标准是已注册的别名与基于路径的结果一致，同时未知格式保留其 fallback。
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描述
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 beimage/jpeg)Audio(data=..., format="mp3")->audio/mp3(should beaudio/mpeg)Audio(data=..., format="m4a")->audio/m4a(should beaudio/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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