[AIRFLOW-1659] Fix invalid obj attribute bug in file_task_handler.py
[AIRFLOW-1659] Fix invalid obj attribute bug in file_task_handler.py

Closes #2645 from jgao54/fix-file_task_handler-bug

Twistbioscience/incubator-airflow,Twistbioscience/incubator-airflow,Twistbioscience/incubator-airflow,Twistbioscience/incubator-airflow