Now we know how a server tells a client what type of information is being returned, but how does the server figure out this information?
In the UNIX and DOS world, files are usually identified using file name suffixes. In the DOS world they are limited to three characters, but on Unix systems they can be any length. Thus a file called london_zoo.gif is likely to be an image in the GIF format. Servers typically have a set of built in suffixes that they assume denote particular content types. They also let you specify the content types of your own suffixes in case you have any local oddities, or something new that the server designer hadn't thought of.
On Apple Macintoshes, most users are used to being able to call files whatever they want, and the resource information associated with the file specifies which application created it and its file type. Although MacHTTP can use the Mac file type to specify the content type, what happens with data that didn't originate on a Mac? Thus, MacHTTP also has a similar configuration file and also lets you use Unix style suffixes to specify the MIME content type to report back to clients.