to_mail (ActionMailer::Part)

    # File lib/action_mailer/part.rb, line 56
        def to_mail(defaults)
          part = TMail::Mail.new

          real_content_type, ctype_attrs = parse_content_type(defaults)

          if @parts.empty?
            part.content_transfer_encoding = transfer_encoding || "quoted-printable"
            case (transfer_encoding || "").downcase
              when "base64" then
                part.body = TMail::Base64.folding_encode(body)
              when "quoted-printable"
                part.body = [Utils.normalize_new_lines(body)].pack("M*")
              else
                part.body = body
            end

            # Always set the content_type after setting the body and or parts!
            # Also don't set filename and name when there is none (like in
            # non-attachment parts)
            if content_disposition == "attachment"
              ctype_attrs.delete "charset"
              part.set_content_type(real_content_type, nil,
                squish("name" => filename).merge(ctype_attrs))
              part.set_content_disposition(content_disposition,
                squish("filename" => filename).merge(ctype_attrs))
            else
              part.set_content_type(real_content_type, nil, ctype_attrs)
              part.set_content_disposition(content_disposition)
            end
          else
            if String === body
              @parts.unshift Part.new(:charset => charset, :body => @body, :content_type => 'text/plain')
              @body = nil
            end

            @parts.each do |p|
              prt = (TMail::Mail === p ? p : p.to_mail(defaults))
              part.parts << prt
            end

            part.set_content_type(real_content_type, nil, ctype_attrs) if real_content_type =~ /multipart/
          end

          headers.each { |k,v| part[k] = v }

          part
        end