def normalize_options(options)
      options = options.dup

      options.tap do |opts|
        opts[:prefix] ||= "gridfs"
        opts[:prefix] = options[:prefix].gsub(/^\//, '')
        opts[:mapper] ||= lambda { |path| %r!^/#{options[:prefix]}/(.+)!.match(path)[1] }
      end
    end
def normalize_options(options)
      options = options.dup

      options.tap do |opts|
        opts[:prefix] ||= "gridfs"
        opts[:prefix] = options[:prefix].gsub(/^\//, '')
        opts[:mapper] ||= lambda { |path| %r!^/#{options[:prefix]}/(.+)!.match(path)[1] }
      end
    end
TODO: doc explanation/example of custom mapper