instance method sort_parts

# File actionmailer/lib/action_mailer/base.rb, line 638
      def sort_parts(parts, order = [])
        order = order.collect { |s| s.downcase }

        parts = parts.sort do |a, b|
          a_ct = a.content_type.downcase
          b_ct = b.content_type.downcase

          a_in = order.include? a_ct
          b_in = order.include? b_ct

          s = case
          when a_in && b_in
            order.index(a_ct) <=> order.index(b_ct)
          when a_in
            -1
          when b_in
            1
          else
            a_ct <=> b_ct
          end

          # reverse the ordering because parts that come last are displayed
          # first in mail clients
          (s * -1)
        end

        parts
      end