extensions/ipynb/src/serializers.ts/getCellMetadata
export function getCellMetadata(cell: NotebookCell | NotebookCellData) {
	return {
		// it contains the cell id, and the cell metadata, along with other nb cell metadata
		...(cell.metadata?.custom ?? {}),
		// promote the cell attachments to the top level
		attachments: cell.metadata?.custom?.attachments ?? cell.metadata?.attachments
	};
}