java IO流下载.docx文件时报文件损坏的错误，求解？

ftp.changeWorkingDirectory(FTP_BASEPATH);// 转移到FTP服务器目录
ftp.enterLocalPassiveMode();
ftp.setFileType(FTP.BINARY_FILE_TYPE);
FTPFile[] fs = ftp.listFiles();

        for (FTPFile ff : fs) {
          String ftpName=  new String(ff.getName().getBytes("ISO8859-1"), "utf-8");
            if (ftpName.equals(fileName)) {
                InputStream in = ftp.retrieveFileStream(ff.getName());//读取ftp服务器文件，返回输入流
                int len = 0;
                byte[] buff = new byte[1024 * 1024];
                response.reset();
                response.setContentType("application/octet-stream");
          
                response.addHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
                OutputStream out = response.getOutputStream();  //响应输出字节流
                while ((len = in.read(buff)) != -1) {
                    log.debug("以字节流形式写出OutPutStream");
                    out.write(buff, 0, len);
                    out.flush();
                }
                in.close();
                out.close();
                ftp.completePendingCommand();
            }

        }

out.flush(); 放在while外面试试