Formidable - edit success message, added method to add post ID as using global post object / wp_query all appear to fail. Attempted to validate post id from hidden input field.

filter.php

<?php
add_filter('frm_success_filter', 'add_the_download_form_button', 1000, 2);
function add_the_download_form_button($type, $form)
{
    // If this is the download form we can now check for a file.
    if ($form->id == 4
        && isset($_POST)) {

        $pid = null;

        //this relies on a hidden input set to post id with format of `pid:<post_id>` I have tried over 1 hour to get global post ID from within this filter and it simply wont work so fuck it.
        foreach ($_POST['item_meta'] as $meta) {
            if (strpos($meta, ':') === 3) {
                $parts = explode(':', $meta);
                if (count($parts) == 2 && $parts[0] === 'pid') {
                    $pid_tmp = (int)end($parts);
                    if (is_int($pid_tmp) && get_post($pid_tmp)) {
                        $pid = $pid_tmp;
                        break;
                    }
                }
            }
        }

        if (isset($pid) && !is_null($pid)) {
            $file = get_field('case_study_file', $pid);
            if ($file) {
                $type = 'message';
                $form->options['success_msg'] .= '<a href="' . $file . '" target="_blank" class="button button--chevron">Download Now</a>';
            }
        }
    }

    return $type;
}