rsync - Jan 2014 - Excluding; iff foo.part exists, exclude foo and foo.part

I'm syncing a download directory, where various programs drop their files. I
would like to exclude partial firefox downloads.

This could be accomplished by excluding "foo" and
"foo.part", iff "foo.part"
exists. How can I accomplish this with exclude rules? Or would I have to
generate the rules dynamically, by some script.