src/vs/workbench/services/languageDetection/browser/languageDetectionWorkerServiceImpl.ts/LanguageDetectionWorkerClient/_guessLanguageIdByUri
class LanguageDetectionWorkerClient:	private _guessLanguageIdByUri(uri: URI): string | undefined {
		const guess = this._languageService.guessLanguageIdByFilepathOrFirstLine(uri);
		if (guess && guess !== 'unknown') {
			return guess;
		}
		return undefined;
	}