How to set thunderbird to correctly attach files

Text files (with Content-Disposition: attachment instead of inline)
Thunderbird has to set "Content-Disposition: inline" by default. It means when you attach a text file to an outgoing e-mail, the contents would be display as part of the email.

"Edit" -> "Preferences" -> "Advanced" -> "Config Editor".
(Using the windows version it's probably "Tools" -> "Options" -> etc.)

set mail.content_disposition_type to 1 (default is 0)