### 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
'label': '10',
            'max_active_tis_per_dag': None,
            'max_retry_delay': None,
            'on_execute_callback': None,
            'on_failure_callback': None,
            'on_retry_callback': None,
            'on_success_callback': None,
            'outlets': [],
            'owner': 'airflow',
            'params': {},
            'pool': 'default_pool',
            'pool_slots': 1,
            'priority_weight': 1,
            'queue': 'default',
            'resources': None,
            'retries': 0,
            'retry_delay': timedelta(0, 300),
            'retry_exponential_backoff': False,
            'run_as_user': None,
            'sla': None,
            'start_date': None,
            'subdag': None,
            'task_id': '10',
            'trigger_rule': 'all_success',
            'wait_for_downstream': False,
            'weight_rule': 'downstream',
        } == fields, """
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     ACTION NEEDED! PLEASE READ THIS CAREFULLY AND CORRECT TESTS CAREFULLY
 Some fields were added to the BaseOperator! Please add them to the list above and make sure that