mml-dnd-attach-options

mml-dnd-attach-options is a variable defined in `mml.el'.
Its value is

nil

Documentation:
Which options should be queried when attaching a file via drag and drop.

If it is a list, valid members are `type', `description' and
`disposition'. `disposition' implies `type'. If it is nil,
don't ask for options. If it is t, ask the user whether or not
to specify options.

You can customize this variable.

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