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MIME Basic Structures and Headers
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Additional MIME Headers
In addition to the five basic headers above, the MIME standard allows additional ones to be defined. The only restriction is that they all must start with the word “Content-”, which again, clearly labels them as describing content of a MIME entity (message or body part). Obviously, both sender and recipient must support a custom header for it to be useful.
Several new MIME headers have in fact been created and documented in various Internet RFCs. Some are actually designed not specifically for use by e-mail messages, but for use by other protocols that make use of MIME technology, such as the HyperText Transfer Protocol (HTTP). Here are three notable ones.
In multipart MIME messages, this header may be given to MIME body parts to control how information is presented to the user. The two most common values are “inline”, which says the content is intended to be displayed automatically along with other body parts, and “attachment”, which indicates that the content is separate from the main document. This header is defined in RFC 2183.
Allows the location of a MIME body part to be identified using a uniform resource identifier (URI). This is sometimes used when encoding HTML and other multimedia-enabled document formats into e-mail using MIME multipart messages. It is defined in RFC 2557.
Specifies the length of a MIME entity in bytes. This header is not commonly used in e-mail applications of MIME, but is an important header in HTTP. It is described in the HTTP standards, first appearing in RFC 1945.
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Key Concept: MIME provides flexibility in the information that can be carried in e-mail messages, by encoding non-ASCII data in ASCII form, and by adding special headers that describe this data and how it is to be interpreted. The most important MIME headers are Content-Type, which describes what sort of data is in the message, and Content-Transfer-Encoding, which specifies how the data is encoded. MIME supports two basic overall formats: simple structure, in which a single type of discrete media is encoded in a message; and complex structure, which encodes a composite media type that can carry multiple kinds of information.
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