This is explained in the section of this document dealing with
   multipart/alternative.

6.2.  Optional Content-Description Header Field

   The ability to associate some descriptive information with a given
   body is often desirable. For example, it may be useful to mark an
   "image" body as "a picture of the Space Shuttle Endeavor."  Such text
   may be placed in the Content-Description header field.

   description := "Content-Description" ":" *text

   The description is presumed to be given in the US-ASCII character
   set, although the mechanism specified in [RFC-1522] may be used for
   non-US-ASCII Content-Description values.

7.    The Predefined Content-Type Values

   This document defines seven initial Content-Type values and an
   extension mechanism for private or experimental types.  Further
   standard types must be defined by new published specifications.  It
   is expected that most innovation in new types of mail will take place
   as subtypes of the seven types defined here.  The most essential
   characteristics of the seven content-types are summarized in Appendix
   F.

7.1  The Text Content-Type

   The text Content-Type is intended for sending material which is
   principally textual in form.  It is the default Content-Type.  A
   "charset" parameter may be used to indicate the character set of the
   body text for some text subtypes, notably including the primary
   subtype, "text/plain", which indicates plain (unformatted) text.  The
   default Content-Type for Internet mail is "text/plain; charset=us-
   ascii".

   Beyond plain text, there are many formats for representing what might
   be known as "extended text" -- text with embedded formatting and
   presentation information.  An interesting characteristic of many such
   representations is that they are to some extent readable even without
   the software that interprets them.  It is useful, then, to
   distinguish them, at the highest level, from such unreadable data as




 
RFC 1521                          MIME                    September 1993


   images, audio, or text represented in an unreadable form.  In the
   absence of appropriate interpretation software, it is reasonable to
   show subtypes of text to the user, while it is not reasonable to do
   so with most nontextual data.

   Such formatted textual data should be represented using subtypes of
   text.  Plausible subtypes of text are typically given by the common
   name of the representation format, e.g., "text/richtext" [RFC-1341].

7.1.1.     The charset parameter

   A critical parameter that may be specified in the Content-Type field
   for text/plain data is the character set.  This is specified with a
   "charset" parameter, as in:

        Content-type: text/plain; charset=us-ascii

   Unlike some other parameter values, the values of the charset
   parameter are NOT case sensitive.  The default character set, which
   must be assumed in the absence of a charset parameter, is US-ASCII.

   The specification for any future subtypes of "text" must specify
   whether or not they will also utilize a "charset" parameter, and may
   possibly restrict its values as well.  When used with a particular
   body, the semantics of the "charset" parameter should be identical to
   those specified here for "text/plain", i.e., the body consists
   entirely of characters in the given charset.  In particular, definers
   of future text subtypes should pay close attention the the
   implications of multibyte character sets for their subtype
   definitions.

   This RFC specifies the definition of the charset parameter for the
   purposes of MIME to be a unique mapping of a byte stream to glyphs, a
   mapping which does not require external profiling information.

   An initial list of predefined character set names can be found at the
   end of this section.  Additional character sets may be registered
   with IANA, although the standardization of their use requires the
   usual IESG [RFC-1340] review and approval.  Note that if the
   specified character set includes 8-bit data, a Content-Transfer-
   Encoding header field and a corresponding encoding on the data are
   required in order to transmit the body via some mail transfer
   protocols, such as SMTP.

   The default character set, US-ASCII, has been the subject of some
   confusion and ambiguity in the past.  Not only were there some
   ambiguities in the definition, there have been wide variations in
   practice.  In order to eliminate such ambiguity and variations in the