I just set this up last week. A few thoughts:
My project is set up as follows:
I used an NPE to receive dropped objects.
Config of the droppable widget:
Note: StatementOrInvoice is a specialization of Files entity. Until I added the highlighted line, I could drag StatementOrInvoice objects but could not drop them.
Details of one of the Droppable Entities configurations below:
And finally, the FileDropMicroflow
Note: in my app, I enabled dragging from one folder to another, so I change the association of the File that is dropped to the NewFolder and recalculate some statistics about both the New and Old folder.
Hope that helps,