response 需要做三个配置：
response.reset();
response.setHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
response.setContentType("application/octet-stream;charset=UTF-8");
response.addHeader("Content-Length", "" + data.length); //这一句需要加上
byte[] buffer = new byte[1024];//每次读取的字节数小点，可以避免打开文件时报“文件已损坏”的问题
