WebM MIME type: video/webm

The MIME type for .webm files is video/webm, and audio/webm for WebM files that carry only sound. Neither is in the IANA registry: the WebM project defines them in its own specification, and browsers and servers have used them from the start, which is why the tool marks video/webm as unofficial. WebM is a restricted form of Matroska, whose own types RFC 9559 registered in 2024 as video/matroska and audio/matroska; WebM keeps its own names.

video/webm   WebM video
.webm
Content-Type: video/webm

Search audio/webm in the tool for the sound-only type and its .weba extension.

The codecs parameter

A WebM file holds VP8, VP9 or AV1 video and Vorbis or Opus audio, and the codecs parameter says which:

<video controls>
  <source src="/clip.webm" type='video/webm; codecs="vp9, opus"'>
  <source src="/clip.mp4" type="video/mp4">
</video>

The browser takes the first source it can play, so the MP4 is the fallback for one that can’t decode the WebM. video.canPlayType('video/webm; codecs="vp9, opus"') answers "probably", "maybe" or an empty string.

MediaRecorder

WebM is the default video format for MediaRecorder in Chrome and Firefox, and the recorder’s mimeType is where the codecs parameter turns up in practice:

const mimeType = MediaRecorder.isTypeSupported('video/webm;codecs=vp9,opus')
  ? 'video/webm;codecs=vp9,opus'
  : 'video/webm';
const recorder = new MediaRecorder(stream, { mimeType });
recorder.ondataavailable = (event) => chunks.push(event.data);
recorder.onstop = () => upload(new Blob(chunks, { type: recorder.mimeType }));

A Blob built that way carries the full string, parameter included, so an upload endpoint that checks the type against a fixed video/webm rejects it. Compare only the part before ;. Safari records MP4 by default, so accept video/mp4 too.

In your stack

nginx’s mime.types has had video/webm webm; since 2011, and Apache’s bundled file, Express 4 and 5, and Python’s mimetypes map .webm to video/webm as well. None of them can tell an audio-only .webm from a video, so give sound-only files .weba or set the type yourself. nginx has no weba line:

# http block, after include mime.types
types {
    audio/webm  weba;
}

AddType video/webm .webm
AddType audio/webm .weba

aws s3 cp clip.webm s3://my-bucket/videos/clip.webm --content-type video/webm
aws s3 cp memo.webm s3://my-bucket/audio/memo.webm --content-type audio/webm