I’ve always viewed contentType to be a superset of mimeType. The only difference being the optional character set encoding. If the contentType does not include an optional character set encoding then it is identical to a mimeType. Otherwise, the mimeType is the data prior to the character set encoding sequence.
E.G. text/html; charset=UTF-8
text/html is the mimeType
; is the additional parameters indicator
charset=UTF-8 is the character set encoding parameter
E.G. application/msword
application/msword is the mimeType
It cannot have a character set encoding as it describes a well formed octet-stream not comprising characters directly.