code: def _retrieve(self, uid):
          
          query = dict(uid=uid)
          doc = self.db.pastes.find_one(query)
          if 'data_id' in doc:
              data_id = doc.pop('data_id')
              gfs = gridfs.GridFS(self.db)
              doc.update(data=gfs.get(data_id).read())
          return doc
docs: Return a dict with the contents of the paste, including the raw
          data, if any, as the key 'data'. Must pass in uid, not shortid.