How to use file_save_upload when in a tabbed form

In a vertical tab form the path to a form input is
$form['vertical_tabs_card_details']['card_image'] = array(

the documentation uses a simple key value for the file_save_upload function
as in
file_save_upload ('card_image');

which does not work for the tabbed field
nor does
file_save_upload("'vertical_tabs_card_details']['card_image'");
or any other values i have tried.

How should it be implemented?