Use this syntax for the file argument:
%Arg_n"prompt"%
or
%(File)Arg_n"prompt"%
This syntax produces an action that:
Accepts a dropped file as the file argument.
Displays a dialog box that prompts for a file name when the user double-clicks the action icon.
For example, this execution string performs lp -oraw on a dropped file. If the action is started by double-clicking the icon, a dialog box appears prompting for the file name.
EXEC_STRING lp -oraw %Arg_1"File to print:"%