pewc-multiple-file-uploads.php

<?php
/**
 * @param $multiple   '' | 'multiple'
 * @param $product_id Product ID
 * @param $id         Field ID
 */

// Enable multiple uploads for all file inputs
function prefix_filter_multiple( $multiple, $product_id, $id ) {
	return 'multiple';
}
add_filter( 'pewc_allow_multiple_file_upload', 'prefix_filter_multiple', 10, 3 );

// Enable multiple file uploads by product ID
function prefix_filter_multiple_by_id( $multiple, $product_id, $id ) {
  if( $product_id == 99 ) {
    return 'multiple';
  }
  return $multiple;
}
add_filter( 'pewc_allow_multiple_file_upload', 'prefix_filter_multiple_by_id', 10, 3 );