How to detect whether a webm file is audio or video?

Question

I am building an uploader with ReactJS. When a user uploads a video or audio file, I take the MIME files and register the type (video or audio) to display icons later on.

I have a problem when they upload audio files with the extension webm (music.webm) and the MIME type is video/webm , so I register it as video, when in fact I should register it as audio.

I have tried to decode the video and find some codec to be sure that there is no video using blob.text() , but I can't find the information, or I don't know where to look.

I know it is possible, because the player knows the difference.

How can I detect whether a webm file is audio or video?