如何发送邮件中文附件名无乱码？

发邮件中发附件那块代码如下：

 private void sendAttachmentHeader(string attachment)
        {
            string contentType = "application/octet-stream";//text/plain;
            string header = "--" + boundary_pro + "\r\n"
                    + "Content-Type:" + contentType + ";;name=" + System.IO.Path.GetFileName(attachment) + "\r\n"
                    + "Content-Transfer-Encoding:base64\r\n"
                    + "Content-Disposition:attachment;filename=\"" + System.IO.Path.GetFileName(attachment) + "\"\r\n";
            socketSend(header + "\r\n");
        }

发出去的邮件收取是如果是中文显示 ???????.txt

如何发送才能正确显示中文附件名？

 attachment.NameEncoding = Encoding.GetEncoding("GB2312");

http://blog.csdn.net/joeblackzqq/article/details/2045680