try this:

Code:
MailMessage mailMessage = new MailMessage();
mailMessage.From = txtTo.Text
mailMessage.To = txtEmail.Text;
mailMessage.Subject = "Attachment"
mailMessage.Body = "Attachment Test";
mailMessage.Attachment.Add(new MailAttachment(txtAttachmentPath1.Text));
mailMessage.Attachment.Add(new MailAttachment(txtAttachmentPath2.Text));
mailMessage.Attachment.Add(new MailAttachment(txtAttachmentPath3.Text));
SmtpMail.Server = "localhost";
SmtpMail.Send(mailMessage);

The above attaches 3 attachments to the outgoing email.

NB. txtAttachmentPath2.Text is the path to the attachment.