Como o candidato a pergunta já se encontrou, de acordo com um relatório de bug antigo , isso é causado por :
When [sending] an email with a PDF document attached, the Content-Type is set to text/plain, while they should be application/pdf
... o que se deve a:
[...] you may have picked up a wrong content-type definition when opening an attachment with such a specification, then making it the default action by checking the box which is in the respective dialog. This associates the extension of that file name with the given content type and will be used for outgoing e-mail from that point on.
... e resolvido por:
Find the
mimeTypes.rdf
file in your profile folder (not the default copy in your installation folders), then rename it while Thunderbird is down. Restarting will create a new file from the default template, and the issue should be gone. You can use the new file if you don't have a significant number of definitions, or replace it with the old one and use the MIME Edit extension to remove the faulty entry.