### 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
},
        "start_date": 1564617600.0,
        '_task_group': {
            '_group_id': None,
            'prefix_group_id': True,
            'children': {'bash_task': ('operator', 'bash_task'), 'custom_task': ('operator', 'custom_task')},
            'tooltip': '',
            'ui_color': 'CornflowerBlue',
            'ui_fgcolor': '#000',
            'upstream_group_ids': [],
            'downstream_group_ids': [],
            'upstream_task_ids': [],
            'downstream_task_ids': [],
        },
        "is_paused_upon_creation": False,
        "_dag_id": "simple_dag",
        "doc_md": "### DAG Tutorial Documentation",
        "fileloc": None,
        "tasks": [
            {
                "task_id": "bash_task",
                "owner": "airflow",
                "retries": 1,
                "retry_delay": 300.0,
                "max_retry_delay": 600.0,
                "sla": 100.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,
                "_outlets": [],