create attachment and set media Type
// see http://msdn.microsoft.com/de-de/library/system.net.mime.mediatypenames.application.aspx
Attachment data = new Attachment(
"PATH_TO_YOUR_FILE",
MediaTypeNames.Application.Octet);
// your path may look like Server.MapPath("~/file.ABC")
message.Attachments.Add(data);
Thanks & regard
Sham :)