title: Using XCOM_PULL in a FileSenor filepath arguement?
body: <p>I am really new to Airflow, so please forgive me if this is a dim question. I did search unsuccessfully on Stackoverlow to find a similar question.</p>
  <p>I have a download stream task that waits for a file to download. I'd like to abstract the hardcoded filepath and retrieve the path, stored in an XCOM.</p>
  <pre><code>  t2 = FileSensor(
      task_id = 'waiting_for_file_download',
      poke_interval = 60 * 5,
      timeout = 60 * 10,
      mode = 'reschedule',
      filepath = {{ ti.xcom_pull(task_ids = 'downloaded_file', key = 'file_path') }} + 'transformed' + '_new.csv.gz'
    )
  </code></pre>
  <p>Is this possible?</p>
  <p>Reading the <a href="http://airflow.apache.org/docs/apache-airflow/1.10.12/_api/airflow/contrib/sensors/file_sensor/index.html?highlight=filesensor#airflow.contrib.sensors.file_sensor.FileSensor.template_fields" rel="nofollow noreferrer">official filesensor documentation</a> did not really help me, as a newcomer.</p>
  <p>I see there are two additional fields 1) templates 2) fs_conn_id</p>
  <p>UPDATE
  Reading the docs I can see a XCOM.get_one() however this is not working either:</p>
  <pre><code>filepath = XCom.get_one(
    execution_date = date.today(),
    dag_id = 'My_DAG',
    task_id = 'downloaded_file',
    key = 'file_path'
  </code></pre>
  <p>I see that other users use this in conjunction with **context, however, I do not know how you can use the context within the FileSenor()?</p>