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The Content-Type header field is used to specify the nature of the data in the body of an entity, by giving type and subtype identifiers, and by providing auxiliary information that may be required for certain types. After the type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The set of meaningful parameters differs for the different types. In particular, there are NO globally-meaningful parameters that apply to all content-types. Global mechanisms are best addressed, in the MIME model, by the definition of additional Content-* header fields. The ordering of parameters is not significant. Among the defined parameters is a "charset" parameter by which the character set used in the body may be declared. Comments are allowed in accordance with RFC 822 rules for structured header fields.
In general, the top-level Content-Type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a Content-Type of "image/xyz" is enough to tell a user agent that the data is an image, even if the user agent has no knowledge of the specific image format "xyz". Such information can be used, for example, to decide whether or not to show a user the raw data from an unrecognized subtype -- such an action might be reasonable for unrecognized subtypes of text, but not for unrecognized subtypes of image or audio. For this reason, registered subtypes of audio, image, text, and video, should not contain embedded information that is really of a different type. Such compound types should be represented using the "multipart" or "application" types.
Parameters are modifiers of the content-subtype, and do not fundamentally affect the requirements of the host system. Although most parameters make sense only with certain content-types, others are "global" in the sense that they might apply to any subtype. For example, the "boundary" parameter makes sense only for the "multipart" content-type, but the "charset" parameter might make sense with several content-types.
An initial set of seven Content-Types is defined by this document. This set of top-level names is intended to be substantially complete. It is expected that additions to the larger set of supported types can generally be accomplished by the creation of new subtypes of these initial types. In the future, more top-level types may be defined only by an extension to this standard. If another primary type is to be used for any reason, it must be given a name starting with "X-" to indicate its non-standard status and to avoid a potential conflict with a future official name.
In the Augmented BNF notation of RFC 822, a Content-Type header field value is defined as follows:
content := "Content-Type" ":" type "/" subtype *(";"
parameter)
; case-insensitive matching of type and subtype
type := "application" / "audio"
/ "image" / "message"
/ "multipart" / "text"
/ "video" / extension-token
; All values case-insensitive
extension-token := x-token / iana-token
iana-token := <a publicly-defined extension token,
registered with IANA, as specified in
Appendix E>
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
subtype := token ; case-insensitive
parameter := attribute "=" value
attribute := token ; case-insensitive
value := token / quoted-string
token := 1*<any (ASCII) CHAR except SPACE, CTLs,
or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@"
/ "," / ";" / ":" / "\" / <">
/ "/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values
Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=", and the removal of ".".
Note also that a subtype specification is MANDATORY. There are no default subtypes.
The type, subtype, and parameter names are not case sensitive. For example, TEXT, Text, and TeXt are all equivalent. Parameter values are normally case sensitive, but certain parameters are interpreted to be case-insensitive, depending on the intended use. (For example, multipart boundaries are case-sensitive, but the "access-type" for message/External-body is not case-sensitive.)
Beyond this syntax, the only constraint on the definition of subtype names is the desire that their uses must not conflict. That is, it would be undesirable to have two different communities using "Content-Type: application/foobar" to mean two different things. The process of defining new content-subtypes, then, is not intended to be a mechanism for imposing restrictions, but simply a mechanism for publicizing the usages. There are, therefore, two acceptable mechanisms for defining new Content-Type subtypes:
The seven standard initial predefined Content-Types are detailed in the bulk of this document. They are:
Default RFC 822 messages are typed by this protocol as plain text in the US-ASCII character set, which can be explicitly specified as "Content-type: text/plain; charset=us-ascii". If no Content-Type is specified, this default is assumed. In the presence of a MIME- Version header field, a receiving User Agent can also assume that plain US-ASCII text was the sender's intent. In the absence of a MIME-Version specification, plain US-ASCII text must still be assumed, but the sender's intent might have been otherwise.
It should be noted that the list of Content-Type values given here may be augmented in time, via the mechanisms described above, and that the set of subtypes is expected to grow substantially.
When a mail reader encounters mail with an unknown Content-type value, it should generally treat it as equivalent to "application/octet-stream", as described later in this document.
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