mime_encode_multipart (TMail::Mail)

    # File lib/action_mailer/vendor/tmail-1.1.0/tmail/net.rb, line 122
        def mime_encode_multipart( top = true )
          self.mime_version = '1.0' if top
          self.set_content_type 'multipart', 'mixed'
          e = encoding(nil)
          if e and not /\A(?:7bit|8bit|binary)\z/i === e
            raise ArgumentError,
                  'using C.T.Encoding with multipart mail is not permitted'
          end
        end