Content-type: text/html; charset=US-ASCII
Content-Disposition: inline
Content-transfer-encoding: quoted-printable

Content-type: text/plain; name="..."
Content-Disposition: attachment; filename="..."
Content-transfer-encoding: base64

//Для отправки почты
        static void SendMail()
        {
            string smtpHost = "хост почтовик";
            int smtpPort = порт;
            string login = "логин";
            string pass = "пароль";

            //connect
            SmtpClient client = new SmtpClient(smtpHost, smtpPort);
            client.Credentials = new NetworkCredential(login, pass);

            //from-to-title-body message
            string from = "test@gmail.com";
            string to = "test1@gmail.com, test2@gmail.com";
            string subject = "Test header";
            string body = "Это тестовая рассылка, проверьте вложение";

            //attachment
            Attachment attData = new Attachment(@"C:test.xlsx");

            //create message and attaching
            MailMessage mess = new MailMessage(from, to, subject, body);
            mess.Attachments.Add(attData);

            try
            {
                client.Send(mess);
                MessageBox.Show("Сообщение отправлено");
            }

            catch (SmtpException ex)
            {
                MessageBox.Show(ex);
            }

        }