http://www.faqs.org/rfcs/rfc2045.html
A MIME-Version header field.
A Content-Type header field, generalized from
RFC 1049, which can be used to specify the media type and subtype of data in the body of a message and to fully specify the native representation (canonical form) of such data.
A Content-Transfer-Encoding header field, which can be used to specify both the encoding transformation that was applied to the body and the domain of the result.
Two additional header fields that can be used to further describe the data in a body, the Content-ID and Content-Description header fields.