### 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
"""
        Assert extra field exists & OperatorLinks defined in Plugins and inbuilt Operator Links.
        This tests also depends on GoogleLink() registered as a plugin
        in tests/plugins/test_plugin.py
        The function tests that if extra operator links are registered in plugin
        in ``operator_extra_links`` and the same is also defined in
        the Operator in ``BaseOperator.operator_extra_links``, it has the correct
        extra link.
        """
        test_date = datetime(2019, 8, 1)
        dag = DAG(dag_id='simple_dag', start_date=test_date)
        CustomOperator(task_id='simple_task', dag=dag, bash_command=["echo", "true"])
        serialized_dag = SerializedDAG.to_dict(dag)
        assert "bash_command" in serialized_dag["dag"]["tasks"][0]
        dag = SerializedDAG.from_dict(serialized_dag)
        simple_task = dag.task_dict["simple_task"]
        assert getattr(simple_task, "bash_command") == ["echo", "true"]
        
        
        
        
        assert serialized_dag["dag"]["tasks"][0]["_operator_extra_links"] == [
            {'tests.test_utils.mock_operators.CustomBaseIndexOpLink': {'index': 0}},
            {'tests.test_utils.mock_operators.CustomBaseIndexOpLink': {'index': 1}},
        ]