def get_file_size(self, fid):
        """
        Gets size of uploaded file
        Or None if file doesn't exist.

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

        Returns:
            Int or None
        """
        url = self.get_file_url(fid)
        res = self.conn.head(url)
        if res is not None:
            size = res.headers.get("content-length", None)
            if size is not None:
                return int(size)
        return None