code: def of_cte(cls, header: Optional[ContentTransferEncodingHeader]) \
              -> 'MessageDecoder':
          
          if header is None:
              return _NoopDecoder()
          hdr_str = str(header).lower()
          custom = cls.registry.get(hdr_str)
          if custom is not None:
              return custom
          elif hdr_str in ('7bit', '8bit'):
              return _NoopDecoder()
          elif hdr_str == 'quoted-printable':
              return _QuotedPrintableDecoder()
          elif hdr_str == 'base64':
              return _Base64Decoder()
          else:
              raise NotImplementedError(hdr_str)
docs: Return a decoder from the CTE header value.
  
          There is built-in support for ``7bit``, ``8bit``, ``quoted-printable``,
          and ``base64`` CTE header values. Decoders can be added or overridden
          with the :attr:`.registry` dictionary.
  
          Args:
              header: The CTE header value.