mp3 files sometimes throw strange mime types as per this answer: https://stackoverflow.com/a/2755288/14482130
If you are doing some user validation do not allow 'application/octet-stream' or 'application/x-zip-compressed' as suggested above since they can contain be .exe or other potentially dangerous files.
In order to validate when mime type gives a false negative you can use fleep as per this answer https://stackoverflow.com/a/52570299/14482130 to finish the validation.