The five discrete top-level media types are:

    (1)   text -- textual information.  The subtype "plain" in
          particular indicates plain text containing no
          formatting commands or directives of any sort. Plain
          text is intended to be displayed "as-is". No special
          software is required to get the full meaning of the
          text, aside from support for the indicated character
          set. Other subtypes are to be used for enriched text in
          forms where application software may enhance the
          appearance of the text, but such software must not be
          required in order to get the general idea of the
          content.  Possible subtypes of "text" thus include any
          word processor format that can be read without
          resorting to software that understands the format.  In
          particular, formats that employ embeddded binary
          formatting information are not considered directly
          readable. A very simple and portable subtype,
          "richtext", was defined in RFC 1341, with a further
          revision in RFC 1896 under the name "enriched".






 
RFC 2046                      Media Types                  November 1996


    (2)   image -- image data.  "Image" requires a display device
          (such as a graphical display, a graphics printer, or a
          FAX machine) to view the information. An initial
          subtype is defined for the widely-used image format
          JPEG. .  subtypes are defined for two widely-used image
          formats, jpeg and gif.

    (3)   audio -- audio data.  "Audio" requires an audio output
          device (such as a speaker or a telephone) to "display"
          the contents.  An initial subtype "basic" is defined in
          this document.

    (4)   video -- video data.  "Video" requires the capability
          to display moving images, typically including
          specialized hardware and software.  An initial subtype
          "mpeg" is defined in this document.

    (5)   application -- some other kind of data, typically
          either uninterpreted binary data or information to be
          processed by an application.  The subtype "octet-
          stream" is to be used in the case of uninterpreted
          binary data, in which case the simplest recommended
          action is to offer to write the information into a file
          for the user.  The "PostScript" subtype is also defined
          for the transport of PostScript material.  Other
          expected uses for "application" include spreadsheets,
          data for mail-based scheduling systems, and languages
          for "active" (computational) messaging, and word
          processing formats that are not directly readable.
          Note that security considerations may exist for some
          types of application data, most notably
          "application/PostScript" and any form of active
          messaging.  These issues are discussed later in this
          document.

   The two composite top-level media types are:

    (1)   multipart -- data consisting of multiple entities of
          independent data types.  Four subtypes are initially
          defined, including the basic "mixed" subtype specifying
          a generic mixed set of parts, "alternative" for
          representing the same data in multiple formats,
          "parallel" for parts intended to be viewed
          simultaneously, and "digest" for multipart entities in
          which each part has a default type of "message/rfc822".







 
RFC 2046                      Media Types                  November 1996


    (2)   message -- an encapsulated message.  A body of media
          type "message" is itself all or a portion of some kind
          of message object.  Such objects may or may not in turn
          contain other entities.  The "rfc822" subtype is used
          when the encapsulated content is itself an RFC 822
          message.  The "partial" subtype is defined for partial
          RFC 822 messages, to permit the fragmented transmission
          of bodies that are thought to be too large to be passed
          through transport facilities in one piece.  Another
          subtype, "external-body", is defined for specifying
          large bodies by reference to an external data source.

   It should be noted that the list of media type values given here may
   be augmented in time, via the mechanisms described above, and that