These headers were introduced in IE 8 and are both related to MIME-handling in IE, so we'll cover them within the same section. MIME-types are used to identify different types of data. Consider what happens when the browser downloads a file from a web server — and keep in mind that a file is just a chunk of bytes. The browser has no idea how to interpret the file unless the server gives it a hint. This is where MIME-types come into play, it lets the server tell the browser just what kind of file it is. If it's a PDF it should for example tell the browser application/pdf and the browser would now how to handle the file.