docs: def register_previewer(self, name, previewer):
          """Register a previewer in the system."""
          if name in self.previewers:
              assert name not in self.previewers, \
                  "Previewer with same name already registered"
          self.previewers[name] = previewer
          if hasattr(previewer, 'previewable_extensions'):
              self._previewable_extensions |= set(
                  previewer.previewable_extensions)