message is actually text in the US-ASCII character set, since it
      might well be a message that, using the conventions that predate
      this document, includes text in another character set or non-
      textual data in a manner that cannot be automatically recognized
      (e.g., a uuencoded compressed UNIX tar file).  Although there is
      no fully acceptable alternative to treating such untyped messages
      as "text/plain; charset=us-ascii", implementors should remain
      aware that if a message lacks both the MIME-Version and the
      Content-Type header fields, it may in practice contain almost
      anything.

   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.

5.    The Content-Transfer-Encoding Header Field

   Many Content-Types which could usefully be transported via email are
   represented, in their "natural" format, as 8-bit character or binary
   data.  Such data cannot be transmitted over some transport protocols.
   For example, RFC 821 restricts mail messages to 7-bit US-ASCII data
   with lines no longer than 1000 characters.




 
RFC 1521                          MIME                    September 1993


   It is necessary, therefore, to define a standard mechanism for re-
   encoding such data into a 7-bit short-line format.  This document
   specifies that such encodings will be indicated by a new "Content-
   Transfer-Encoding" header field.  The Content-Transfer-Encoding field
   is used to indicate the type of transformation that has been used in
   order to represent the body in an acceptable manner for transport.

   Unlike Content-Types, a proliferation of Content-Transfer-Encoding
   values is undesirable and unnecessary.  However, establishing only a
   single Content-Transfer-Encoding mechanism does not seem possible.
   There is a tradeoff between the desire for a compact and efficient
   encoding of largely-binary data and the desire for a readable
   encoding of data that is mostly, but not entirely, 7-bit data.  For
   this reason, at least two encoding mechanisms are necessary: a
   "readable" encoding and a "dense" encoding.

   The Content-Transfer-Encoding field is designed to specify an
   invertible mapping between the "native" representation of a type of
   data and a representation that can be readily exchanged using 7 bit
   mail transport protocols, such as those defined by RFC 821 (SMTP).
   This field has not been defined by any previous standard. The field's
   value is a single token specifying the type of encoding, as
   enumerated below.  Formally:

   encoding := "Content-Transfer-Encoding" ":" mechanism

   mechanism :=     "7bit"  ;  case-insensitive
                  / "quoted-printable"
                  / "base64"
                  / "8bit"
                  / "binary"
                  / x-token

   These values are not case sensitive.  That is, Base64 and BASE64 and
   bAsE64 are all equivalent.  An encoding type of 7BIT requires that
   the body is already in a seven-bit mail-ready representation.  This
   is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is
   assumed if the Content-Transfer-Encoding header field is not present.

   The values "8bit", "7bit", and "binary" all mean that NO encoding has
   been performed. However, they are potentially useful as indications
   of the kind of data contained in the object, and therefore of the
   kind of encoding that might need to be performed for transmission in
   a given transport system.  In particular:

       "7bit" means that the data is all represented as short
            lines of US-ASCII data.





 
RFC 1521                          MIME                    September 1993


       "8bit" means that the lines are short, but there may be
            non-ASCII characters (octets with the high-order
            bit set).

       "Binary" means that not only may non-ASCII characters
            be present, but also that the lines are not
            necessarily short enough for SMTP transport.

   The difference between "8bit" (or any other conceivable bit-width
   token) and the "binary" token is that "binary" does not require