code: def add_header(self, _name, _value, **_params):
          
          parts = []
          for k, v in _params.items():
              if v is None:
                  parts.append(k.replace('_', '-'))
              else:
                  parts.append(_formatparam(k.replace('_', '-'), v))
          if _value is not None:
              parts.insert(0, _value)
          self[_name] = SEMISPACE.join(parts)
docs: Extended header setting.
  
          name is the header field to add.  keyword arguments can be used to set
          additional parameters for the header field, with underscores converted
          to dashes.  Normally the parameter will be added as key="value" unless
          value is None, in which case only the key will be added.  If a
          parameter value contains non-ASCII characters it can be specified as a
          three-tuple of (charset, language, value), in which case it will be
          encoded according to RFC2231 rules.  Otherwise it will be encoded using
          the utf-8 charset and a language of ''.
  
          Examples:
  
          msg.add_header('content-disposition', 'attachment', filename='bud.gif')
          msg.add_header('content-disposition', 'attachment',
                         filename=('utf-8', '', 'Fußballer.ppt'))
          msg.add_header('content-disposition', 'attachment',
                         filename='Fußballer.ppt'))