docs: def get_attachment_data(cls, session, attachment_id):
          """Return a specific attachment's data.
  
          Args:
              session (requests.sessions.Session): Authenticated session.
              attachment_id (int): The ID of the attachment from which to get
                  data.
  
          Returns:
              helpscout.models.AttachmentData: An attachment data singleton, if
                  existing. Otherwise ``None``.
          """
          return cls(
              '/attachments/%d/data.json' % attachment_id,
              singleton=True,
              session=session,
              out_type=AttachmentData,
          )