instance method next_file_name

# File actionmailer/lib/action_mailer/vendor/tmail-1.2.7/tmail/mailbox.rb, line 127
    def next_file_name
      unless n = @last_file
        n = 0
        Dir.entries(@dirname)\
                .select {|s| /\A\d+\z/ === s }\
                .map {|s| s.to_i }.sort\
        .each do |i|
          next unless FileTest.file? "#{@dirname}/#{i}"
          n = i
        end
      end
      begin
        n += 1
      end while FileTest.exist? "#{@dirname}/#{n}"
      @last_file = n

      "#{@dirname}/#{n}"
    end