Thanks! I've tried a couple of different ways and they all return zero results:

var file_results = search.create({
			type: 'file',
			filters: [
				search.createFilter({
					name: 'folder',
					operator: <http://search.Operator.IS|search.Operator.IS>,
					values: folderId
				})
			]
		}).run();
		log.debug({
			title: 'file_results',
			detail: JSON.stringify(file_results)
		});

var file_results = search.create({
			type: 'folder',
			filters: [
				search.createFilter({
					name: 'internalid',
					operator: <http://search.Operator.IS|search.Operator.IS>,
					values: folderId
				})
			],
			columns: [
				search.createColumn({
					name: 'internalid',
					join: 'file'
				})
			]
		}).run();
		log.debug({
			title: 'file_results',
			detail: JSON.stringify(file_results)
		});