Certain browsers, try to determine the content type and encoding of the response even when these properties are defined correctly. This can make the web application vulnerable against Cross-Site Scripting (XSS) attacks. E.g. the Internet Explorer and Safari treat responses with the content type text/plain as HTML, if they contain HTML tags.
The only defined value, "nosniff", prevents Internet Explorer and Google Chrome from MIME-sniffing a response away from the declared content-type. This also applies to Google Chrome, when downloading extensions. This reduces exposure to drive-by download attacks and sites serving user uploaded content that, by clever naming, could be treated by MSIE as executable or dynamic HTML files.