mh-media-type-regexp

mh-media-type-regexp is a variable defined in `mh-mime.el'.
Its value is

"\\(a\\(?:pplication\\|udio\\)\\|image\\|m\\(?:essage\\|ultipart\\)\\|text\\|video\\)/[-.+a-zA-Z0-9]+"

Documentation:
Regexp matching valid media types used in MIME attachment compositions.