java写的文件下载接口，客户端无法获取到文件大小问题

直接上代码： 

        // 字符串转InputStream
        byte[] manifestPlistBytes = manifestPlistStr.getBytes("UTF-8");
        InputStream input = new ByteArrayInputStream(manifestPlistBytes);

        String filename = System.currentTimeMillis() + "manifest.plist";

        int len = 0;
        byte[] bytes = new byte[1024];
        ServletOutputStream servletOutputStream = resp.getOutputStream();

        resp.setContentType("application/octet-stream");

        resp.setHeader("Accept-Ranges", "bytes");
        resp.setHeader("Content-Length", String.valueOf(manifestPlistBytes.length));

        resp.setHeader("Content-Disposition", "attachment; filename=" + filename);

        resp.setCharacterEncoding("UTF-8");

        while ((len = input.read(bytes)) > 0) {
            servletOutputStream.write(bytes, 0, len);
        }
        servletOutputStream.close();
        input.close();

关键代码：

        resp.setHeader("Accept-Ranges", "bytes");
        resp.setHeader("Content-Length", String.valueOf(manifestPlistBytes.length));

参考网上资料和jfinal源码。