I have found the following on this (http://www.tek-tips.com/viewthread.cfm?qid=1175419) page:
Maybe this helps us to improve that tool.
Code:
CGI programs can return a myriad of document types.
They can send back an image to the client, and HTML document, a plaintext document, or perhaps even an audio clip.
They can also return references to other documents.
The client must know what kind of document you're sending it so it can present it accordingly.
In order for the client to know this, your CGI program must tell the server what type of document it is returning
Maybe this helps us to improve that tool.