src/vs/editor/common/services/languageService.ts/LanguageService/guessLanguageIdByFilepathOrFirstLine
class LanguageService:	public guessLanguageIdByFilepathOrFirstLine(resource: URI | null, firstLine?: string): string | null {
		const languageIds = this._registry.guessLanguageIdByFilepathOrFirstLine(resource, firstLine);
		return firstOrDefault(languageIds, null);
	}