(RFC 1494 published August 1993, subtype last updated August 1993)

The application/x400-bp content-type

This content-type is defined to carry any X.400(88) body part for
which there is no registered IANA mapping.

    The content-type field is

      application/x400-bp

    The parameters are:

          bp-type=<INTEGER or OBJECT IDENTIFIER>

The body contains the raw ASN.1 IPM.body octet stream, including the
initial tag octet.

If the body is a basic body part, the bp-type parameter is set to the
number of the body part's context-specific tag, that is, the tag of
the IPMS.Body.BodyPart component.

If the body is an Extended Body Part, the bp-type parameter is set to
the OBJECT IDENTIFIER from

         IPMS.body.externally-defined.data.direct-reference

No attempt is made to turn the parameters of Extended Body Parts into
MIME parameters.  (This task is the responsibility of the recipient's
UA).

For example, a basic VideotexBodyPart will have

   Content-type=application/x400-bp; bp-type=6

whilst a Extended Videotex body part will have

   Content-type=application/x400-bp; bp-type=2.6.1.4.5

application/x400-bp will need a content-transfer-encoding of base64
or quoted-printable when carried in 7-bit MIME.  Since there is no
way to know beforehand the content, it is recommended to just inspect
the first 1 KByte or so of data and choose the one that seems to
produce the more compact encoding.

If this is not feasible, Base64 is recommended.