I have vm-honor-content-disposition set to [the default] nil. 
As of rev 1307 on the trunk, I get nothing when I hit 'return' on a text/plain button that has:
After editing the message manually to have:
... I can now get the attachment displayed inline in the presentation buffer when I hit 'return' on the mime button.
 Actually it doesn't matter if I set that var to nil, t, or internal-only. I still can't display the attachment if the disposition is 'attachment'.