docs: def attach_related(self, filename=None, content=None, mimetype=None):
          """
          Attaches a file with the given filename and content. The filename can
          be omitted and the mimetype is guessed, if not provided.
  
          If the first parameter is a MIMEBase subclass it is inserted directly
          into the resulting message attachments.
          """
          if isinstance(filename, MIMEBase):
              assert content == mimetype == None
              self.related_attachments.append(filename)
          else:
              assert content is not None
              self.related_attachments.append((filename, content, mimetype))