src/vs/workbench/contrib/notebook/common/notebookProvider.ts/NotebookProviderInfo/possibleFileEnding
class NotebookProviderInfo:	static possibleFileEnding(selectors: NotebookSelector[]): string | undefined {
		for (const selector of selectors) {
			const ending = NotebookProviderInfo._possibleFileEnding(selector);
			if (ending) {
				return ending;
			}
		}
		return undefined;
	}