http://www.ietf.org/rfc/rfc2183.txt
This memo provides a mechanism whereby messages conforming to the MIME specifications [RFC 2045, RFC 2046, RFC 2047, RFC 2048, RFC 2049] can convey presentational information. It specifies the "Content-Disposition" header field, which is optional and valid for any MIME entity ("message" or "body part"). Two values for this header field are described in this memo; one for the ordinary linear presentation of the body part, and another to facilitate the use of mail to transfer files. It is expected that more values will be defined in the future, and procedures are defined for extending this set of values.

This document is intended as an extension to MIME. As such, the reader is assumed to be familiar with the MIME specifications, and [RFC 822]. The information presented herein supplements but does not replace that found in those documents.

This document is a revision to the Experimental protocol defined in RFC 1806. As compared to RFC 1806, this document contains minor editorial updates, adds new parameters needed to support the File Transfer Body Part, and references a separate specification for the handling of non-ASCII and/or very long parameter values.