code: def dropbox_post_factory(request):
      
      try:
          max_age = int(request.registry.settings.get('post_token_max_age_seconds'))
      except Exception:
          max_age = 300
  
      try:
          drop_id = parse_post_token(
              token=request.matchdict['token'],
              secret=request.registry.settings['post_secret'],
              max_age=max_age)
      except SignatureExpired:
          raise HTTPGone('dropbox expired')
      except Exception:  # don't be too specific on the reason for the error
          raise HTTPNotFound('no such dropbox')
      dropbox = request.registry.settings['dropbox_container'].get_dropbox(drop_id)
      if dropbox.status_int >= 20:
          raise HTTPGone('dropbox already in processing, no longer accepts data')
      return dropbox
docs: receives a UUID via the request and returns either a fresh or an existing dropbox
      for it