BufferedReader reader = new BufferedReader(
                    new FileReader(servletContext.getRealPath("XLStemplate/reportXLS.xlsx")));

            //response.setContentType("application/octet-stream; charset=UTF-8");
            PrintWriter out = new PrintWriter(new OutputStreamWriter(response.getOutputStream(), "UTF8"), true);

            // Read and write 4K chars at a time
            // (Far more efficient than reading and writing a line at a time)
            char[] buf = new char[4 * 1024]; // 4Kchar buffer
            int len;
            while ((len = reader.read(buf, 0, buf.length)) != -1) {
                out.write(buf, 0, len);
            }
            out.flush();
            out.close();

response.sendRedirect("XLStemplate/XLStemplate.xlsx");

String fileExtension ="application/x-www-form-urlencoded";
if (extension.equals("pdf"))
{
    sURLforRequest = GWT.getHostPageBaseURL() + "requiredPDF";
    fileExtension = "application/pdf";
}
else if (extension.equals("xls"))
{
    sURLforRequest = GWT.getHostPageBaseURL() + "requiredXLS";
    fileExtension = "application/x-www-form-urlencoded";
}

RequestBuilder builder = new RequestBuilder(RequestBuilder.POST, sURLforRequest);
//this is important
builder.setHeader("Content-type", fileExtension);
//builder.setHeader("Content-type", "application/x-www-form-urlencoded");
//builder.setHeader("Content-type", "application/octet-stream");

builder.sendRequest(sData, new RequestCallback() {
    @Override
    public void onResponseReceived(Request request, Response response) {
        //TODO: replace with servlet link
        Window.open(sURLforRequest, "_parent", "location=no");

    }
    @Override
    public void onError(Request request, Throwable exception) {
        MessageBox.alert("Request error","The page doesn't open", null);
    }
});