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Using the MIME-Version, Content-Type, and Content-Transfer-Encoding
header fields, it is possible to include, in a standardized way,
arbitrary types of data objects with RFC 822 conformant mail
messages. No restrictions imposed by either RFC 821 or RFC 822 are
violated, and care has been taken to avoid problems caused by
additional restrictions imposed by the characteristics of some
Internet mail transport mechanisms (see
Appendix B). The "multipart"
and "message" Content-Types allow mixing and hierarchical structuring
of objects of different types in a single message. Further Content-Types
provide a standardized mechanism for tagging messages or body
parts as audio, image, or several other kinds of data. A
distinguished parameter syntax allows further specification of data
format details, particularly the specification of alternate character
sets. Additional optional header fields provide mechanisms for
certain extensions deemed desirable by many implementors. Finally, a
number of useful Content-Types are defined for general use by
consenting user agents, notably message/partial, and
message/external-body.
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