I notice gnus-mime-copy-part (`c' on the button) looks for a filename
parameter from mm-handle-type, but I wonder if that ought to be
mm-handle-disposition.

Certainly gnus itself seems to give the filename under
Content-Disposition rather than Content-Type, following rfc2183 if I
read it right.

Fixing this lets normal-mode do the right thing in the new buffer.

	* gnus-art.el (gnus-mime-copy-part): Look for filename
	parameter under content-disposition, not content-type.