src/vs/workbench/services/textfile/common/textFileEditorModel.ts/TextFileEditorModel/onFilesAssociationChange
class TextFileEditorModel:	private onFilesAssociationChange(): void {
		if (!this.isResolved()) {
			return;
		}

		const firstLineText = this.getFirstLineText(this.textEditorModel);
		const languageSelection = this.getOrCreateLanguage(this.resource, this.languageService, this.preferredLanguageId, firstLineText);

		this.modelService.setMode(this.textEditorModel, languageSelection);
	}