This dialog pops-up when you do the following:
Added in version 2.1: "Check" button, press it to see a list of filename from the dropped files.
This dialog is almost the same as the Add Dialog accept that you cannot select the files you want to add.
You can't use wildcards as well.
Another different thing is a directory box with "Add from directory:", in that box should the root directory of all dropped files if you want them to be added relatively to that directory. This will work like the "From Current Dir" in the normal add dialog.