Here is the sample code.
System.IO.MemoryStream ms = new System.IO.MemoryStream();
System.IO.StreamWriter writer = new System.IO.StreamWriter(ms);
writer.Write("Hello its my sample file");
writer.Flush();
writer.Dispose();
ms.Position = 0;
System.Net.Mime.ContentType ct = new System.Net.Mime.ContentType(System.Net.Mime.MediaTypeNames.Text.Plain);
System.Net.Mail.Attachment attach = new System.Net.Mail.Attachment(ms, ct);
attach.ContentDisposition.FileName = "myFile.txt";
// I guess you know how to send email with an attachment
// after sending email
ms.Close();
Edit 1
You can specify other file types by System.Net.Mime.MimeTypeNames like System.Net.Mime.MediaTypeNames.Application.Pdf
Based on Mime Type you need to specify correct extension in FileName for instance "myFile.pdf"