Use a call to clearwin_string@('DROPPED_FILE') within the call-back in order to
get the name of the file that has been dropped. If there is more than one file
then the call-back function is called for each in turn. You can use
clearwin_info@('DROPPED_COUNT') to get the number of files that have been
dropped (call this n say) and then use clearwin_info@('DROPPED_CURRENT')
to get the index of the current file in the range from 1 to n.