### 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
"ui_color": "#f0ede4",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command', 'env'],
                "template_fields_renderers": {'bash_command': 'bash', 'env': 'json'},
                "bash_command": "echo {{ task.task_id }}",
                'label': 'bash_task',
                "_task_type": "BashOperator",
                "_task_module": "airflow.operators.bash",
                "pool": "default_pool",
                "executor_config": {
                    '__type': 'dict',
                    '__var': {
                        "pod_override": {
                            '__type': 'k8s.V1Pod',
                            '__var': PodGenerator.serialize_pod(executor_config_pod),
                        }
                    },
                },
                "doc_md": "### Task Tutorial Documentation",
            },
            {
                "task_id": "custom_task",
                "retries": 1,
                "retry_delay": 300.0,
                "max_retry_delay": 600.0,
                "sla": 100.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_is_dummy": False,
                "_outlets": [],