Update class-wc-admin-cpt-product.php

Adding a new filter to the process_product_file_download_paths() method so that plugins can intercept the file list and decided whether permissions should be updated. This will be a huge help to sites that sometimes change downloadable file names but don't want existing permissions to be reset. A plugin can parse the list and remove files from the $downloadable_files array if a particular file should not have permissions reset.