### 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)

airflow/models/baseoperator.py
template_ext: Iterable[str] = ()
    
    template_fields_renderers: Dict[str, str] = {}
    
    ui_color = '#fff'  
    ui_fgcolor = '#000'  
    pool = ""  
    
    _base_operator_shallow_copy_attrs: Tuple[str, ...] = (
        'user_defined_macros',
        'user_defined_filters',
        'params',
        '_log',
    )
    
    shallow_copy_attrs: Tuple[str, ...] = ()
    
    operator_extra_links: Iterable['BaseOperatorLink'] = ()
    
    __serialized_fields: Optional[FrozenSet[str]] = None
    _comps = {
        'task_id',
        'dag_id',
        'owner',
        'email',
        'email_on_retry',
        'retry_delay',
        'retry_exponential_backoff',
        'max_retry_delay',