how to download file in gae
public class dumpDataServlet extends HttpServlet {
    public void service(HttpServletRequest req, HttpServletResponse resp) throws IOException {
        String msg = "";
        boolean dumpFile = false;
        JSONObject json = null;
        resp.setCharacterEncoding(Constant.RESPONSE_CHARACTER_ENCODING);
        try {
            try {
                msg = req.getParameter("dumpFile") + " ";
                dumpFile = Boolean.valueOf(req.getParameter("dumpFile"));
            } catch (Exception ex) {
            }json = getFromOtherGAE();
            if (dumpFile) {
                OutputStream out = resp.getOutputStream();
                this.parseExcelFile(json).write(out); //parseExcelFile will return a HSSFWorkbook
                out.flush();
                out.close();
            } else {
                resp.getWriter().println(json.toString());
            }
        } catch (Exception ex) {
            msg = ex.getClass() + " " + ex.getMessage();
            resp.getWriter().println(msg);
        }
    }
}

resp.setContentType("application/vnd.ms-excel");
resp.setHeader("Content-Disposition","attachment; filename=download.xls");