Set offset of file index.

        Args:
            file_idx_offset: It can be either an integer or 'auto'. If set
                to an integer, the filename will start from
                ``file_idx_offset`` + 1. If set to ``'auto'``, the filename
                will start from existing max file index plus 1.

def set_file_idx_offset(self, file_idx_offset=0):
        """Set offset of file index.

        Args:
            file_idx_offset: It can be either an integer or 'auto'. If set
                to an integer, the filename will start from
                ``file_idx_offset`` + 1. If set to ``'auto'``, the filename
                will start from existing max file index plus 1.
        """
        if isinstance(file_idx_offset, int):
            self.file_idx_offset = file_idx_offset
        elif file_idx_offset == 'auto':
            self.file_idx_offset = self.storage.max_file_idx()
        else:
            raise ValueError('"file_idx_offset" must be an integer or `auto`')