dired-dnd-protocol-alist is a variable defined in `dired.el'.
Its value is

(("^file:///" . dired-dnd-handle-local-file) ("^file://" . dired-dnd-handle-file) ("^file:" . dired-dnd-handle-local-file))

The functions to call when a drop in `dired-mode' is made.
See `dnd-protocol-alist' for more information. When nil, behave
as in other buffers. Changing this option is effective only for
new Dired buffers.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.