When you serve a document from a Web server, you might want to immediately prompt the user to save the file directly to the user’s disk, without opening it in the browser. However, for known MIME (Multipurpose Internet Mail Extensions) types such as Microsoft Word (“application/ms-word”), the default behavior is to open the document in Internet Explorer.

You can use the content-disposition header to override this default behavior. Its format is:

Content-disposition: attachment; filename=fname.ext

[http://support.microsoft.com/kb/260519/EN-US/]