I found this property to add into your windows in the Schema with the description 'If true, the window accepts drag and drops.' It works well enough, and dragging a file over the window gives the satisfying add file image for the cursor. However, I'm totally lost as to what event would be fired or what win32 API function I would have to use in order to make use of this information. Does anyone have any ideas or hints?

Try searching first, the drag and drop feature has no event in Plus itself yet last time i looked but here's a solution.

Drag and drop