MIME headers appear at the beginning of a MIME message as well as within the separate body parts. Some MIME headers can be used both as message headers and in MIME body parts. Some additional headers are defined for use only in body parts.
The following headers are defined in MIME:
There are additional headers that are applied in specialized situations, such as Content-Base and Content-Location. All of the "Content-xxx" headers have defined sub-headers, fields, and/or parameters. Headers that begin with "Content-" are the only headers that have defined meaning in body parts. All other body part headers can be ignored and might actually be removed by message transfer agents (MTAs).
MIME-Version is a required header indicating that this message is to use the rules of MIME. "MIME-Version: 1.0" is the only currently defined MIME-Version header allowed. The MIME-Version header is a top-level header only and does not appear in body parts unless the body part is an encapsulated, fully formed message of content-type message/rfc822, which might have its own MIME-Version header.
Content-Type headers are used to specify the media type and subtype of data in the body of a message and to fully specify the native representation of such data. This header embodies much of the power of MIME. The IETF can add new official content types. Additionally, private content-type values can be defined by anyone. Such private content types have values of "x-something" or "X-something", where "something" can take on any value.
Content-Transfer-Encoding headers can have two different meanings. If the value is "base64" or "quoted-printable", then the header indicates the encoding used for this body part. If the value is "7bit", "8bit", or "binary", then the header indicates that there is no encoding and that this value indicates only the type of content this body part contains. The default is "7bit". It should be noted that "8bit" and "binary" cannot be assured of being handled by all Internet (SMTP) MTAs, and are not actually valid in Internet mail. Provision is made for private Content-Transfer-Encoding headers. These have values that begin with "x-" or "X-". These are for specialized cases where the users have the tools to decode or otherwise process a specific "x-" encoding.
Content-ID headers are world-unique values that identify body parts, individually or as groups. They are necessary at times to distinguish body parts and allow cross-referencing between body parts.
Content-Description headers are optional and are often used to add descriptive text to non-textual body parts.
Content-Disposition headers provide information about how to present a message or a body part. When a body part is to be treated as an attached file, the Content-Disposition header will include a file name parameter.