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9. Additional MIME Header Fields

MIME reserves the right to define additional header fields whose names begin with the string "Content-". These header fields are intended to provide additional meta-information about the MIME entity body.

Common Additional Header Fields​

Content-Disposition​

While not part of RFC 2045, Content-Disposition (defined in RFC 2183) is a commonly used additional header field:

Content-Disposition: inline
Content-Disposition: attachment; filename="document.pdf"

Other Extensions​

Mail system implementors may define private header fields beginning with "Content-", but such fields should use the "X-" prefix to avoid conflicts with future standards:

X-Content-Custom-Field: value

Key Points:

  • Fields beginning with "Content-" are reserved for MIME use
  • Private extensions should use "X-" prefix
  • Implementations must ignore unrecognized Content-* fields
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