This type is discussed briefly in Chapter 11. A typical example is a mail message consisting of an HTML document, along with all the images that appear inline in the document. In this case, the mail message must contain all the parts, but must also support URLs that link between the parts, so that the document can contain URLs referencing the images lying in the same message. The browser uses this content-base to locate all the related parts, including the image file included as the second part of the message. This mechanism is largely used by the Netscape Navigator 4 mail client, when composing HTML mail messages.
Comments are in italics.
See RFC 2110 for details. For the server to be able to send a content-type header, it must know the type of the data being returned. For example, files with the extension .mpeg are usually assumed to be MPEG movies, while files ending in .html are assumed to be HTML documents, and so on. Of course, it is then up to the author, when placing documents on the server, to ensure that the documents use the extensions corresponding to each document's content. If a browser receives a file from an HTTP server, it is explicitly told the content-type by the server's response header.