https://greenbytes.de/tech/webdav/rfc7231.html#media.type

The type, subtype, and parameter name tokens are case-insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. The presence or absence of a parameter might be significant to the processing of a media-type, depending on its definition within the media type registry.

...it then continues showing examples that imply that the charset value is case-insensitive (which is correct), but it would be helpful if it cited RFC 2046 which defines charset's case sensitivity.