### 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
from airflow.models.dag import DAG, create_timetable
from airflow.providers_manager import ProvidersManager
from airflow.serialization.enums import DagAttributeTypes as DAT, Encoding
from airflow.serialization.helpers import serialize_template_field
from airflow.serialization.json_schema import Validator, load_dag_schema
from airflow.settings import json
from airflow.timetables.base import Timetable
from airflow.utils.code_utils import get_python_source
from airflow.utils.module_loading import as_importable_string, import_string
from airflow.utils.task_group import TaskGroup
try:
    
    from kubernetes.client import models as k8s
    from airflow.kubernetes.pod_generator import PodGenerator
    
    HAS_KUBERNETES = True
except ImportError:
    HAS_KUBERNETES = False
if TYPE_CHECKING:
    from airflow.ti_deps.deps.base_ti_dep import BaseTIDep
log = logging.getLogger(__name__)
_OPERATOR_EXTRA_LINKS: Set[str] = {
    "airflow.operators.trigger_dagrun.TriggerDagRunLink",
    "airflow.sensors.external_task.ExternalTaskSensorLink",
    
    "airflow.operators.dagrun_operator.TriggerDagRunLink",
    "airflow.sensors.external_task_sensor.ExternalTaskSensorLink",
}
@cache
def get_operator_extra_links():