Thunderbird adding attachments in strange ways?

I'm having a problem sending attachements in Thunderbird.  When I send 
them to folks using other mail clients (most notably Mail on OSX) they 
often get a whole bunch of garbled characters in the mail message.

I believe this is because when Tunderbird is sending the message the 
MIME header looks like:

--------------070309030902090002090008
Content-Type: text/richtext; x-mac-type="5738424E"; 
x-mac-creator="4D535744";
  name="CascadiaOverview.doc"
Content-Transfer-Encoding: base64
Content-Disposition: inline;
  filename="doc.doc"

I noticed that many other mail programs use a Content-Type that starts 
with application/ and have the Content-Disposition be "attachment" 
rather than "inline"

I am currently using version 1.0.2, but I upgraded from 0.9

Other folks in my group don't seem to have this problem.  Does anyone 
have any ideas of what's going on?  Is there some strange option I have 
turned on that is doing this?

     Thanks,
     --Will