docs: def from_mime_type(cls, mime_type):
          """
          Essentially a copy constructor.
           Type.from_mime_type(plaintext)
          is equivalent to:
            t = Type.new(plaintext.content_type.dup)
            t.extensions  = plaintext.extensions.dup
            t.system      = plaintext.system.dup
            t.encoding = plaintext.encoding.dup
          """
          mt = cls(deepcopy(mime_type.content_type))
          mt.extensions = map(deepcopy, mime_type.extensions)
          mt.url = mime_type.url and map(deepcopy, mime_type.url) or None
          mt.system = deepcopy(mime_type.system)
          mt.encoding = deepcopy(mime_type.encoding)
          mt.docs = deepcopy(mime_type.docs)
  
          mt.is_obsolete = mime_type.is_obsolete
          mt.registered = mime_type.is_registered
          return mt