I believe I've found the root cause of the issue; I don't think the feature is working correctly in the public beta (which I installed to see if it would fix some issues with DP3/4), however upon installing DP5, the feature seems to be working OK so far (file has uploaded).
I will let you know soon if the recipient can open the file in the link shortly.
So if you have the PB then, at least in my experience, there is no Mail Drop.