### 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
super().__setattr__(key, value)
        if self._lock_for_execution:
            
            return
        if self.__instantiated and key in self.template_fields:
            
            
            
            
            self.set_xcomargs_dependencies()
    def add_inlets(self, inlets: Iterable[Any]):
        """Sets inlets to this operator"""
        self._inlets.extend(inlets)
    def add_outlets(self, outlets: Iterable[Any]):
        """Defines the outlets of this operator"""
        self._outlets.extend(outlets)
    def get_inlet_defs(self):
        """:return: list of inlets defined for this operator"""
        return self._inlets
    def get_outlet_defs(self):