MIMEheaders

MIME headers are a set of header fields used to describe the content of messages formatted according to the Multipurpose Internet Mail Extensions (MIME) standard. They accompany the body of messages and describe type, encoding, disposition, and intended handling of each part. They are used in email and in other protocols that transport MIME content.

Each header line has the form field-name: field-body. Field-names are case-insensitive, and long header lines may be folded over multiple lines by starting the continuation lines with whitespace. The header area uses ASCII text. Parameter values in the field-body can be quoted.

Common headers describe how a message is to be interpreted. MIME-Version indicates the MIME version (usually 1.0). Content-Type specifies a media type and optional parameters such as boundary or charset; it is used to describe the content of a part or the whole message. Content-Transfer-Encoding indicates how the data is encoded for transport (7bit, 8bit, binary, base64, quoted-printable). Content-Disposition expresses whether the content is inline or an attachment and can provide a filename parameter. Content-ID can be used to reference a part uniquely. Content-Language or Content-Location provide metadata about language or resource location.

Body part structure often relies on multipart types. In multipart messages, a Content-Type header with a multipart subtype (e.g., multipart/alternative, multipart/mixed) uses a boundary parameter to separate parts. Each part has its own set of headers, commonly including its own Content-Type and Content-Transfer-Encoding.

Usage and interoperability: MIME headers may appear in email messages, as well as in HTTP responses where a Content-Type and related parameters are used. Parsing is defined in RFC 2045-2049 and related specifications. Headers must handle folded lines and parameter quoting and are defined to be ASCII-friendly for reliable transport and processing.