A MIME type describes the ‘type of file’ that will be downloaded.
When a request is made to a server, part of the server response is to send the MIME information in the header of the file to be downloaded.
For each file/content type, a server must be configured to send an appropriate MIME type. If this is not done, both the server and the web browser will ‘guess’, and it may appear as if the webpage is broken, or that a plug-in is required to view the content.
reformulation of HTML as an application of XML, in other words an XHTML webpage should be served as a different type of file than an HTML webpage. (Unfortunately Internet Explorer 6.xx ‘chokes’ on the XML MIME type: