Add support for JMP

[kercsoj]

This PR fixes an issue where JMP files were incorrectly identified as audio/mpeg by adding the proper magic byte sequence.

There is no publicly available official documentation for the JMP file format, so I contacted JMP Technical Support for guidance. The magic byte sequence and the recommended MIME type come from them. I also downloaded several JMP sample files from https://community.jmp.com/ from different years to verify that the implementation works correctly.