It would be better if each of the term/foo-win.el files had something
like:

(defvar drag-n-drop-function 'find-file)

(defun foo-drag-n-drop (event)
...
   (mapc drag-n-drop-function (car (cdr (cdr event))))
...
)

so that vm-mail-mode etc. could say:

(set (make-local-variable 'drag-n-drop-function)
      (lambda (file)
        (vm-mime-attach-file file (or (vm-mime-default-type-from-filename file)
                                      "application/octet-stream"))))

instead of hacking foo-drag-n-drop as below.

Thanks,
-- 
Kevin Rodgers