It sounds like drag and drop and file browse both work fine. It's when you test and uncover this edge case is where it fails. What I mean by that is, the user here has chosen the file browse option. But now they have changed their mind, but not fully by dismissing the dialog and drag dropping but rather they attempt to drag a file from the file browse dialog into the drag and drop target?
I'm not sure how typical or realistic this use case is, but if you feel you might have uncovered an edge case that needs addressing it will likely require an SR to fully investigate this.