5. Content-Type Header Field

The purpose of the Content-Type field is to describe the data contained in the body fully enough that the receiving user agent can pick an appropriate agent or mechanism to present the data to the user, or otherwise deal with the data in an appropriate manner. The value in this field is called a media type.

HISTORICAL NOTE: The Content-Type header field was first defined in RFC 1049. RFC 1049 used a simpler and less powerful syntax, but one that is largely compatible with the mechanism given here.

The Content-Type header field specifies the nature of the data in the body of an entity by giving media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. After the media type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute=value notation. The ordering of parameters is not significant.

In general, the top-level media type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a media 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 text, image, audio, and video should not contain embedded information that is really of a different type. Such compound formats should be represented using the "multipart" or "application" types.

Parameters are modifiers of the media subtype, and as such do not fundamentally alter the nature of the content. The set of meaningful parameters depends on the media type and subtype. Most parameters are associated with a single specific subtype. However, a given top-level media type may define parameters which are applicable to any subtype of that type. Parameters may be required by their defining content type or subtype or they may be optional. MIME implementations must ignore any parameters whose names they do not recognize.

For example, the "charset" parameter is applicable to any subtype of "text", while the "boundary" parameter is required for any subtype of the "multipart" media type.

There are NO globally-meaningful parameters that apply to all media types. Mechanisms that might be thought of as global in MIME are best addressed in the MIME model by defining additional Content-* header fields.

An initial set of seven top-level media types is defined in RFC 2046. Five of these are discrete types whose contents are opaque as far as MIME processing is concerned. The remaining two are composite types whose contents require additional MIME processing.

This set of top-level media types 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 a standards-track extension to this standard. If another top-level 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.

5.1. Syntax of the Content-Type Header Field

In the Augmented BNF notation of RFC 822, a Content-Type header field value is defined as follows:

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

5.2. Content-Type Defaults

If a Content-Type header field is not present in an entity, the default Content-Type depends on the context:

• At the top level of a MIME message, the default is text/plain; charset=us-ascii
• Within a body part of a multipart entity, the default Content-Type is text/plain; charset=us-ascii
• Within a "message/rfc822" entity, the default Content-Type is text/plain; charset=us-ascii

---

Key Points:

• Content-Type format: type/subtype; parameter=value
• Type and subtype are case-insensitive
• Parameter names are case-insensitive, but values are usually case-sensitive
• Default value: text/plain; charset=us-ascii

Seven Top-Level Media Types:

Discrete Types

1. text: Textual data
2. image: Image data
3. audio: Audio data
4. video: Video data
5. application: Application data

Composite Types

6. message: Encapsulated message
7. multipart: Multiple parts