### 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
self,
        serialized_schedule_interval,
        expected_timetable,
    ):
        """Test DAGs serialized before 2.2 can be correctly deserialized."""
        serialized = {
            "__version": 1,
            "dag": {
                "default_args": {"__type": "dict", "__var": {}},
                "_dag_id": "simple_dag",
                "fileloc": __file__,
                "tasks": [],
                "timezone": "UTC",
                "schedule_interval": serialized_schedule_interval,
            },
        }
        SerializedDAG.validate_schema(serialized)
        dag = SerializedDAG.from_dict(serialized)
        assert dag.timetable == expected_timetable
    @parameterized.expand(
        [
            (relativedelta(days=-1), {"__type": "relativedelta", "__var": {"days": -1}}),
            (relativedelta(month=1, days=-1), {"__type": "relativedelta", "__var": {"month": 1, "days": -1}}),
            
            (relativedelta(weekday=FR), {"__type": "relativedelta", "__var": {"weekday": [4]}}),
            
            (relativedelta(weekday=FR(2)), {"__type": "relativedelta", "__var": {"weekday": [4, 2]}}),
        ]
    )
    def test_roundtrip_relativedelta(self, val, expected):