### 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
"""
        Additional Properties are disabled on DAGs. This test verifies that all the
        keys in DAG.get_serialized_fields are listed in Schema definition.
        """
        dag_schema: dict = load_dag_schema_dict()["definitions"]["dag"]["properties"]
        
        ignored_keys: set = {
            "is_subdag",
            "tasks",
            "has_on_success_callback",
            "has_on_failure_callback",
            "dag_dependencies",
        }
        keys_for_backwards_compat: set = {
            "_concurrency",
        }
        dag_params: set = set(dag_schema.keys()) - ignored_keys - keys_for_backwards_compat
        assert set(DAG.get_serialized_fields()) == dag_params
    def test_operator_subclass_changing_base_defaults(self):
        assert (
            BaseOperator(task_id='dummy').do_xcom_push is True
        ), "Precondition check! If this fails the test won't make sense"
        class MyOperator(BaseOperator):