### 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
"""Loads DAGs from a module for test."""
    dagbag = DagBag(module_path)
    return dagbag.dags
def make_simple_dag():
    """Make very simple DAG to verify serialization result."""
    with DAG(
        dag_id='simple_dag',
        default_args={
            "retries": 1,
            "retry_delay": timedelta(minutes=5),
            "max_retry_delay": timedelta(minutes=10),
            "depends_on_past": False,
            "sla": timedelta(seconds=100),
        },
        start_date=datetime(2019, 8, 1),
        is_paused_upon_creation=False,
        access_control={"test_role": {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT}},
        doc_md="### DAG Tutorial Documentation",
    ) as dag:
        CustomOperator(task_id='custom_task')
        BashOperator(
            task_id='bash_task',
            bash_command='echo {{ task.task_id }}',
            owner='airflow',