3.7 Media Types

HTTP uses Internet Media Types [17] in the Content-Type (Section 14.17) and Accept (Section 14.1) header fields in order to provide open and extensible data typing and type negotiation.


= type "/" subtype *( ";" parameter )


= token


= token

Parameters MAY follow the type/subtype in the form of attribute/value pairs (as defined in Section 3.6).

The type, subtype, and parameter attribute names are case- insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. Linear white space (LWS) MUST NOT be used between the type and subtype, nor between an attribute and its value. 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. Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations SHOULD only use media type parameters when they are required by that type/subtype definition.

Media-type values are registered with the Internet Assigned Number Authority (IANA [19]). The media type registration process is outlined in RFC 1590 [17]. Use of non-registered media types is discouraged.

3.7.1 Canonicalization and Text Defaults

Internet media types are registered with a canonical form. An entity-body transferred via HTTP messages MUST be represented in the appropriate canonical form prior to its transmission except for "text" types, as defined in the next paragraph.

When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire entity-body. HTTP applications MUST accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).

If an entity-body is encoded with a content-coding, the underlying data MUST be in a form defined above prior to being encoded.

The "charset" parameter is used with some media types to define the character set (Section 3.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value. See Section 3.4.1 for compatibility problems.

3.7.2 Multipart Types

MIME provides for a number of "multipart" types -- encapsulations of one or more entities within a single message-body. All multipart types share a common syntax, as defined in Section 5.1.1 of RFC 2046 [40], and MUST include a boundary parameter as part of the media type value. The message body is itself a protocol element and MUST therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 2046, the epilogue of any multipart message MUST be empty; HTTP applications MUST NOT transmit the epilogue (even if the original multipart contains an epilogue). These restrictions exist in order to preserve the self-delimiting nature of a multipart message- body, wherein the "end" of the message-body is indicated by the ending multipart boundary.

In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. The one exception is the "multipart/byteranges" type (Appendix 19.2) when it appears in a 206 (Partial Content) response, which will be interpreted by some HTTP caching mechanisms as described in Sections 13.5.4 and 14.16. In all other cases, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message- body do not have any significance to HTTP beyond that defined by their MIME semantics.

In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".

Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 [15].