    def flatten(base_path, file_ext)
      Dir["#{base_path}/**/*.#{file_ext}"].each do |file|
        # get the filename and parent dir
        filename = file.tr('/', '-')

        # copy the files to a single temp directory.
        debug("copying #{file} to #{@tmp_dir}/#{filename} ...")
        FileUtils.cp(file, "#{@tmp_dir}/#{filename}")
      end
    end