Describe the bug
isMimeType is not matching valid MIME types that have an underscore in their name.

Examples

• audio/ip-mr_v2.5
• application/media_control+xml
  are valid MIME types, but currently isMimeType will return false

Full original IANA List of MIME Types can be found here, or take a look at the "extracted" list I created out of it, with just the MIME types to make it easier to copy/paste and test:
https://gist.github.com/pano9000/87919bee80b4fbc14b8b29a7dc9878cd

A PR to fix this will follow soon