Vijay,
It seems your's is an
Struts application. As you are using action class,
add this line in your action class,
response.addHeader("Content-disposition","attachment; filename = myFilename.xls");
before in_mapping.findForward();
Note that this will work only if you are forwarding the request through action class and not directly to the html page.
Feel free to get more details if needed.