problem with thunderbird attachments .doc-file association

[Hugo Coolens]

When I have an e-mail with a docx-document as an attachment, thunderbird 
automatically opens LibreOffice when I click the attachment, when I have
however a doc-document I get a an error when clicking on the attachment,
like this:
/tmp/OPDOC24082005-1.doc could not be opened, because the associated 
helper application does not exist. Change the association in your 
preferences.

So I'd like to add the .doc file extension and have LibreOffice open it 
too but I can't find out how to do it in Thunderbirds 
Preferences-->Attachments.
I'm missing an add Content Type-possibility. Maybe someone here knows 
which file I should change manually and how to add a rule for 
.doc-documents?

regards,
hugo

p.s. 1 I'm using Kubuntu Natty Narwal
p.s. 2 When I right click on the attachment-document I don't get a drop 
down box, so I can't choose an application that way neither