In this case, save the attachment to disk and use decoding software to obtain the image file in usable form. In this case, Netscape will assume that encoded data may exist in the message and may or may not succeed in finding and decoding such information. In such a case, saving the message to disk and applying decoding software may work. Use a search engine. I did in researching this document.
RFC documents are the last word on all Internet issues.
Using a search engine you can find detailed documents on every aspect of the Internet. In most cases, though, the basic ideas I have presented should serve. The server will supply part of the HTTP response header but the CGI program must supply the rest as needed. The CGI application must send data or redirect the browser. The first example above is a case in which data will follow.