How to use the netlify-cms-lib-util.fileExtension function in netlify-cms-lib-util

files.map(({ key, name, id, size, path, queryOrder, displayURL, draft }) => {
        const ext = fileExtension(name).toLowerCase();
        return {
          key,
          id,
          name,
          path,
          type: ext.toUpperCase(),
          size,
          queryOrder,
          displayURL,
          draft,
          isImage: IMAGE_EXTENSIONS.includes(ext),
          isViewableImage: IMAGE_EXTENSIONS_VIEWABLE.includes(ext),
        };
      });

return files.filter(file => {
      const ext = fileExtension(file.name).toLowerCase();
      return IMAGE_EXTENSIONS.includes(ext);
    });
  };