Multipart Body Parts

The MIME standard defines several multipart content types that describe the relationship of child body parts, rather than content contained directly in the body part; for example, if a message contains both a US-ASCII text message and an attachment, the top-level body part content type is multipart/mixed. Both the text and the attachment then exist as child body parts below this parent part. Another example is a multipart/alternative content type that indicates alternative representations of a single message; for example, in both US-ASCII text and as HTML formatted text.

For more information, see MIME Extensions to Internet Mail in the appendix.