Media type parsing has some problems with double-quoted parameters:

• ; and , are interpreted as parameter/media type separator even when they occur in a quoted parameter value
• ' is interpreted as a quoting character, the spec says only " must be used for quoting
• Only spaces are allowed, not \t.
• Spaces are allowed in too many places, e.g. type / subtype or parameter = value
• Start and end quotes are removed from quoted parameter values, but the escaped characters inside are not unescaped.
• Quotes are not added back in toString(), which makes deserialization fail if one of the parameters included an escaped double-quote (e.g. type/subtype; param="\"").

Performance can also be improved, as we create many String instances via trim(), substring(), or split().

The clearest explanation of the media type grammar seems to be in
https://tools.ietf.org/html/rfc7231#section-3.1.1.1
https://tools.ietf.org/html/rfc7230#section-3.2.6