### Apache Airflow version

2.1.3 (latest released)

### Operating System

Debian Buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Managed Services (Astronomer, Composer, MWAA etc.)

### Deployment details

Running `quay.io/astronomer/ap-airflow:2.1.3-buster` Docker image

### What happened

In the task attributes view of a BashOperator, the `template_ext` row doesn't show any value, while I would expect to see `('.sh', '.bash')`.

![image](https://user-images.githubusercontent.com/6249654/131467817-8620f68d-0f3d-491d-ad38-cbf5b91cdcb3.png)


### What you expected to happen

Expect to see correct `template_ext` value

### How to reproduce

Run a BashOperator and browse to Instance Details --> scroll down to `template_ext` row

### Anything else

Spent a little time trying to figure out what could be the reason, but haven't found the problem yet. Therefore created issue to track.

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/models/baseoperator.py
**Example**: to run this task in a specific docker container through
        the KubernetesExecutor ::
            MyOperator(...,
                executor_config={
                    "KubernetesExecutor":
                        {"image": "myCustomDockerImage"}
                }
            )
    :type executor_config: dict
    :param do_xcom_push: if True, an XCom is pushed containing the Operator's
        result
    :type do_xcom_push: bool
    :param doc: Add documentation or notes to your Task objects that is visible in
        Task Instance details View in the Webserver
    :type doc: str
    :param doc_md: Add documentation (in Markdown format) or notes to your Task objects
        that is visible in Task Instance details View in the Webserver
    :type doc_md: str
    :param doc_rst: Add documentation (in RST format) or notes to your Task objects
        that is visible in Task Instance details View in the Webserver
    :type doc_rst: str
    :param doc_json: Add documentation (in JSON format) or notes to your Task objects
        that is visible in Task Instance details View in the Webserver
    :type doc_json: str
    :param doc_yaml: Add documentation (in YAML format) or notes to your Task objects
        that is visible in Task Instance details View in the Webserver
    :type doc_yaml: str
    """
    
    template_fields: Iterable[str] = ()