code: def file_exists(self, fid):
          
          res = self.get_file_size(fid)
          if res is not None:
              return True
          return False
docs: Checks if file with provided fid exists
  
          Args:
              **fid**: File identifier <volume_id>,<file_name_hash>
  
          Returns:
              True if file exists. False if not.