mml-dnd-protocol-alist

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

(("^file:///" . mml-dnd-attach-file)
 ("^file://" . dnd-open-file)
 ("^file:" . mml-dnd-attach-file))

Documentation:
The functions to call when a drop in `mml-mode' is made.
See `dnd-protocol-alist' for more information. When nil, behave
as in other buffers.

You can customize this variable.

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