### 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)

tests/serialization/test_dag_serialization.py
"""
        This test verifies that there are no new fields added to BaseOperator. And reminds that
        tests should be added for it.
        """
        base_operator = BaseOperator(task_id="10")
        fields = base_operator.__dict__
        assert {
            '_BaseOperator__instantiated': True,
            '_dag': None,
            '_downstream_task_ids': set(),
            '_inlets': [],
            '_log': base_operator.log,
            '_outlets': [],
            '_upstream_task_ids': set(),
            '_pre_execute_hook': None,
            '_post_execute_hook': None,
            'depends_on_past': False,
            'do_xcom_push': True,
            'doc': None,
            'doc_json': None,
            'doc_md': None,
            'doc_rst': None,
            'doc_yaml': None,
            'email': None,
            'email_on_failure': True,
            'email_on_retry': True,
            'end_date': None,
            'execution_timeout': None,
            'executor_config': {},
            'inlets': [],