### 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/serialization/serialized_objects.py
elif k == "timetable":
                v = decode_timetable(v)
            elif k in cls._decorated_fields:
                v = cls._deserialize(v)
            
            setattr(dag, k, v)
        
        
        
        if "timetable" in encoded_dag:
            dag.schedule_interval = dag.timetable.summary
        else:
            dag.timetable = create_timetable(dag.schedule_interval, dag.timezone)
        
        if "_task_group" in encoded_dag:
            dag._task_group = SerializedTaskGroup.deserialize_task_group(  
                encoded_dag["_task_group"], None, dag.task_dict
            )
        else:
            
            
            dag._task_group = TaskGroup.create_root(dag)
            for task in dag.tasks:
                dag.task_group.add(task)
        
        if "has_on_success_callback" in encoded_dag:
            dag.has_on_success_callback = True
        if "has_on_failure_callback" in encoded_dag:
            dag.has_on_failure_callback = True
        keys_to_set_none = dag.get_serialized_fields() - encoded_dag.keys() - cls._CONSTRUCTOR_PARAMS.keys()