You should likely file a bug.
If wanting to add attachments to TB via DnD then the mimetype of what you're attaching needs to be in TB's .desktop on the MimeType= line.
So for text attachments then this would need to be there or added to end of the line
To try - open the .desktop & edit the MimeType= line
sudo -H gedit /usr/share/applications/thunderbird.desktop
Note that this will open a new compose window w/ the file attached so would need to be the 1st action to be useful, ie. DnD the attachment, then compose the message, ect.