### 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
"_operator_extra_links": [{"tests.test_utils.mock_operators.CustomOpLink": {}}],
                "ui_color": "#fff",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command'],
                "template_fields_renderers": {},
                "_task_type": "CustomOperator",
                "_task_module": "tests.test_utils.mock_operators",
                "pool": "default_pool",
                'label': 'custom_task',
            },
        ],
        "schedule_interval": {"__type": "timedelta", "__var": 86400.0},
        "timezone": "UTC",
        "_access_control": {
            "__type": "dict",
            "__var": {
                "test_role": {
                    "__type": "set",
                    "__var": [permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT],
                }
            },
        },
        "edge_info": {},
        "dag_dependencies": [],
    },
}
ROOT_FOLDER = os.path.realpath(
    os.path.join(os.path.dirname(os.path.realpath(__file__)), os.pardir, os.pardir)
)
def make_example_dags(module_path):