instance method mime_encode_multipart

# File actionmailer/lib/action_mailer/vendor/tmail-1.2.7/tmail/net.rb, line 120
    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