"Arbitrary files in Databricks Repos", allowing not just notebooks to be added to repos, is in Public Preview. I've tried to activate it following the instructions in the above link but the option doesn't appear in Admin Console. Minimum requirements are stated as "you must be running Databricks Runtime 8.4 or above". My cluster is running Runtime 10.3.

I have successfully integrated with my repos in Github but only notebook files are shown.

Repos section of Admin Console (with no "Files in Repos" toggle):

Repos screenshot showing I can only create a folder or a notebook (not a file):

How do I enable this feature?