Checks if file with provided fid exists

        Args:
            **fid**: File identifier <volume_id>,<file_name_hash>

        Returns:
            True if file exists. False if not.

def file_exists(self, fid):
        """Checks if file with provided fid exists

        Args:
            **fid**: File identifier <volume_id>,<file_name_hash>

        Returns:
            True if file exists. False if not.
        """
        res = self.get_file_size(fid)
        if res is not None:
            return True
        return False