src/vs/workbench/services/textfile/browser/textFileService.ts/AbstractTextFileService/suggestFilename
class AbstractTextFileService:	suggestFilename(languageId: string, untitledName: string) {
		const languageName = this.languageService.getLanguageName(languageId);
		if (!languageName) {
			return untitledName;
		}

		const extension = this.languageService.getExtensions(languageId)[0];
		if (extension) {
			if (!untitledName.endsWith(extension)) {
				return untitledName + extension;
			}
		}

		const filename = this.languageService.getFilenames(languageId)[0];
		return filename || untitledName;
	}