Hi @mr-dan
I believe you can add a hidden input file using the “html_after_fields” option. These pages should give you more idea about it: https://www.advancedcustomfields.com/resources/acf_form/, http://www.w3schools.com/TAGs/att_input_type.asp. After that, you can use the $_POST variable to get the data. This page should give you more idea about it: http://php.net/manual/en/reserved.variables.post.php.
I hope this helps 🙂
