Response OutputStream 输出图片

try{
    int reply;
    ftpClient.connect(ip,Integer.parseInt(port));
    ftpClient.login(userName,passWord);
    ftpClient.setBufferSize(1024);
    ftpClient.setFileType(FTPClient.BINARY_FILE_TYPE);
    reply = ftpClient.getReplyCode();
    if(!....){
    ....从ftp服务器上下载图片文件 到本地
    }
    File localFile = new File(localPath+"/"+fileName);
    OutputStream os = new FileOutputStream(localFile);
    if(ftpClient.retrieveFile(fileName,os)){
        //下载到本地硬盘上的图片是能显示的
        response.reset();
        String name = "test.jpg";
        response.reset();
        response.setHeader("Content-Disposition",....);
        response.setContentType("image/jpg");
        String str = os.toString();
        ServletOutputStream sos = response.getOutputStream();
        sos.write(str.getBytes());
        sos.flush();
        sos.close();
        //错误描述：
        ftp下载到本地磁盘，图片能正常显示
        通过response，返回给浏览器，再点击下载或者查看，就无法正常显示，但是下载下来的文件是有字节的。求大神指导一下，该如何处理？
    }
} 

using (var memoryStream = fc.GetFileMemoryStream(fileName))
{
memoryStream.Position = 0;
//bytes = new byte[(int)memoryStream.Length];
byte[] bytes = new byte[1024 * 4];
int count = memoryStream.Read(bytes, 0, bytes.Length);
while (count > 0)
{
byte[] outBuffer = new byte[count];
Array.Copy(bytes, outBuffer, count);
context.Response.BinaryWrite(outBuffer);
count = memoryStream.Read(bytes, 0, bytes.Length);
}
}

读多少字节，输出多少字节