请教ajax如何使用输出流下载文件？(AJAX+Java)

JS部分：用的dojo的
dojo.xhrPost({
        url: "../servlet/Handle",

        content:{"action":"download","filePath":filePath,"fileName":fileName}
});


servlet部分：
            response.reset();
            response.setCharacterEncoding("UTF-8");
            response.setContentType("application/octet-stream");
            response.setHeader("Content-Disposition", "attachment; filename="
                    + fileName);

            bis = new BufferedInputStream(new FileInputStream(filePath));
            bos = new BufferedOutputStream(response.getOutputStream());
            byte[] buff = new byte[2048];
            int bytesRead;
            while (-1 != (bytesRead = bis.read(buff, 0, buff.length)))
            {
                bos.write(buff, 0, bytesRead);
            }