CREATE POLICY "attachments_select_authenticated" ON storage.objects FOR SELECT
  TO authenticated USING (bucket_id = 'attachments');
CREATE POLICY "attachments_insert_own" ON storage.objects FOR INSERT
  TO authenticated WITH CHECK (bucket_id = 'attachments' AND (storage.foldername(name))[1] = auth.uid()::text);
CREATE POLICY "attachments_delete_own" ON storage.objects FOR DELETE
  TO authenticated USING (bucket_id = 'attachments' AND owner = auth.uid());