Unfortunately, LW 5.1.2 doesn't have that option...at least not directly.
There is the option of checking a box that says you will "only share files selected at that point"...or you can opt to also share all new files downloaded to library.
It's an either/or type of choice.
After your post, I changed it to only share files at that point, but not sure if this will help since incomplete files don't show up as being shared anyways.
I'll post back after a few days to let everyone know if this changed the problem...or not.
Thanks!