Send attachments as real attachments

Thunderbird normally sends plain text attachments inline, as part of the message body. If your recipient wants them to appear as real attachments add user_pref("mail.content_disposition_type", 1); to prefs.js or user.js . That should work unless the recipient configures thier email client to display attachments inline. There is nothing you can do about that other than tell them its thier own fault.