MIME types are used to distinguish what sort of operations and attributes a file has. The operations are usually things like: how to open files of a given mime-type, how to view them, what icon is used to display it and so on.
Use the IIS Server Management Console to set the MIME type as follows:
Open IIS Management Console.
Right click on the domain.
Click on the HTTP Headings tab.
Click on the File Types button.
In the Associated extensions window add the file type i.e. ( .rtf, .doc, .xml, etc...)
Next add the correct content type heading