"MIME (Multipurpose Internet Mail Extensions) is a standard to send stuff other than 7Bit ASCII by e-mail." (Claus Aßmann)
The standard is composed of three parts:
- Part One: Format of Internet Message Bodies -- RFC 2045 [L1 ]
- Part Two: Media Types -- RFC 2046 [L2 ]
- Part Three: Message Header Extensions for Non-ASCII Text -- RFC 2047 [L3 ]
It started as an extension to RFC 822 [L4 ], which describes internet mail, but has a much broader usage.
DKF: It's particularly important for smtp and http, but finds good use with nntp (though not usually the attachment parts there).