title: problem to add resources from files in serverless file
body: <p>I am trying to import extra resources in the following way</p>
  <p>**</p>
  <p>resources:</p>
  <ul>
  <li>${file(resources/custom.yml)}</li>
  <li>${file(resources/iam.yml)}</li>
  <li>${file(resources/iot.yml)}</li>
  <li>${file(resources/s3.yml)}</li>
  </ul>
  <p>**</p>
  <p>However, I always get the error</p>
  <p><strong>TypeError [ERR_INVALID_ARG_TYPE]: The &quot;path&quot; argument must be of type string. Received undefined.</strong></p>
  <p>I tried to make changes in the path using quotes and relative or absolute PATH but nothing worked. any ideas?</p>
  <p>I checked official serverless docs that show this done as I did it.</p>
  <p>If someone could point me to the error, please?</p>