Struts 2.0 Download pop up not appearing in browser

Question

I am application is based on struts and in one of the module I am generating reports in pdf and excel format .The reports gets generated in system directory but download pop up dosen't appear on browser .

This is my .xml mapping of action

<action name="report" class="reportAction" method="generateReport">
        <result name="success" type="stream">
            <param name="contentType">application/octet-stream</param>
            <param name="inputName">fileInputStream</param>
            <param name="contentDisposition">attachment;filename=${filename}</param>
            <param name="bufferSize">1024</param>
        </result>
        <result name="error">
        </result>
    </action>

this is my cal to action from jsp :-

function fetchReport()
{
//alert("I am here");
        $.ajax({
               type : "POST",
               url  : "report.action",
               data : jQuery("#parameterGrid").serialize(),
               async : false,
            success : function(dataJSP) {
                //alert("Hi");
                //alert(dataJSP);
                /* jQuery("#showReportDIV").load("Please wait...").html(
                        dataJSP); */

                return true; 
            }
        });
    }

Now I have changed my button on whose click event I generate the report to :-

<sj:submit button="true" 
                    theme="simple" onclick= "fetchReport();" key="Generate Report" />

now the pop up appears but a flat file is download ,I have to browse to open it either in excel or pdf