getting "file in not in correct format that you have specified in the file extension,please verify it is not corrupted and it is from trusted source" message when opening XLS file in IE9..

here am using following code to prompt user to open or save XLS file..when they say open a pop box comming up by saying  "file in not in correct format that you have specified  in the file extension,please verify it is not corrupted and it is from trusted source"..its working fine in IE8 but not in IE9..
could any one can let me know what could be the cause of this issue..
            Response.Buffer = true;
            Response.ClearContent();
            Response.ClearHeaders();
            Response.ContentType = "application/force-download";
            Response.AddHeader("content-disposition","attachment;filename=filename.xls");
            Response.Charset = String.Empty;
...