changing default email client only half works

So within Mail.app I've set the default email program to be Thunderbird. This works fine if for example I click on a "mail to:" link on a web page. But if I right-click a document on my desktop and select "new email with attachment", Mail.app opens up a compose window instead of Thunderbird. Is this the right behaviour or have I done something wrong?

 

That's odd, I didn't add the service myself. Maybe it is something that Quicksilver installed--I recently upgraded to v1.0 (it's great). Otherwise, would adding a service like this be something that you script yourself?

Googling "new email with attachment" and Thunderbird, I found several people asking the same question and no easy solution.