I am trying to configure file based authentication for Kibana users but is it supported? The documentation suggests managing the users through API but is file realm supported? Based on my testing, it is not working but want to confirm 
It is not, there are no Kibana users, all user authentication is done in Elasticsearch.
When you use the native realm you can use Kibana to add, edit or remove users as specified in the documentation.
But if you use the File based authentication this is not possible as mentioned in the documentation.
As the administrator of the cluster, it is your responsibility to ensure the same users are defined on every node in the cluster. The Elastic Stack security features do not deliver any mechanism to guarantee this. You should also be aware that you cannot add or manage users in the file realm via the user APIs and you cannot add or manage them in Kibana on the Management / Security / Users page