See RFC2045 ยง
5.1:
content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; Matching of media type and subtype
; is ALWAYS case-insensitive.
type := discrete-type / composite-type
discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token
composite-type := "message" / "multipart" / extension-token
extension-token := ietf-token / x-token
ietf-token := <An extension token defined by a
standards-track RFC and registered
with IANA.>
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
subtype := extension-token / iana-token
iana-token := <A publicly-defined extension token. Tokens
of this form must be registered with IANA
as specified in RFC 2048.>
parameter := attribute "=" value
attribute := token
; Matching of attributes
; is ALWAYS case-insensitive.
value := token / quoted-string
token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values