string message ="my255CharText";
using (System.IO.StreamWriter file = new System.IO.StreamWriter(@"\shareddatatempbuoy.txt"))
{
    file.WriteLine(message);
}
MailMessage emailMsg = new MailMessage();
emailMsg.To.Add("email@destination.com);
emailMsg.Subject = "my Subject";
emailMsg.From = new MailAddress("email@source.com");
emailMsg.BodyEncoding = System.Text.Encoding.ASCII;
emailMsg.IsBodyHtml = false;

Attachment data = new Attachment(@"\shareddatatempbuoy.txt", MediaTypeNames.Text.Plain);

emailMsg.Attachments.Add(data);
ContentDisposition dispositon = data.ContentDisposition;
dispositon.Inline=true;
SmtpClient smtp = new SmtpClient("SMTP_SERVER");
smtp.Send(emailMsg);

Attachment data = new Attachment(message, MediaTypeNames.Text.Plain);