The Content-Disposition Header

The Content-Disposition header indicates how an application should present the body. A value of inline indicates that the part body should be presented as part of the message. A value of attachment indicates that the part body should be presented as an attached file. In Using the MIME Headers Effectively, the Content-Disposition header contains the value inline. The body should be presented as part of the message.

RFC 1806 defines the Content-Disposition header.