Add header for System.Net.Mail.Attachement class

Question

I need to send email with attachement using standart .Net classes. Attachement should be auto-generated string. I use for this method Attachment.CreateAttachmentFromString(), but when I recieve this message attachcment part doesn't contain a header 'content-disposition' and on some Smtp clients (Mail.app for example) it is part of message(not attachement). How can I add this header for attachement part?