code: def change_object_content_type(self, obj, new_ctype, guess=False):
          
          return self.manager.change_object_content_type(self, obj, new_ctype,
                  guess=guess)
docs: Copies object to itself, but applies a new content-type. The guess
          feature requires this container to be CDN-enabled. If not, then the
          content-type must be supplied. If using guess with a CDN-enabled
          container, new_ctype can be set to None. Failure during the put will
          result in an exception.