Only .yaml files are loaded, not .yml

Files using the .yml file extension are not picked up by the RBAC backend, even though they are valid YAML files and that is a valid YAML file extension.

2021-05-11 15:33:16,873 INFO [-] Loading role definitions from "/opt/stackstorm/rbac/roles/"
2021-05-11 15:33:16,873 INFO [-] Loading user role assignments from "/opt/stackstorm/rbac/assignments/"
2021-05-11 15:33:16,874 INFO [-] Loading group to role map definitions from "/opt/stackstorm/rbac/mappings/"
2021-05-11 15:33:16,874 INFO [-] Synchronizing roles...

After renaming to use .yaml:

2021-05-11 15:33:36,197 INFO [-] Loading role definitions from "/opt/stackstorm/rbac/roles/"
2021-05-11 15:33:36,197 INFO [-] Loading user role assignments from "/opt/stackstorm/rbac/assignments/"
2021-05-11 15:33:36,197 INFO [-] Loading group to role map definitions from "/opt/stackstorm/rbac/mappings/"
2021-05-11 15:33:36,197 DEBUG [-] Loading group to role mapping from: /opt/stackstorm/rbac/mappings/stormers.yaml
2021-05-11 15:33:36,263 INFO [-] Synchronizing roles...

Inspect the RBAC backend path that discovers and loads role, assignment, and mapping files, focusing on how YAML extensions are filtered. Reproduce the behavior with a .yml file alongside the shown .yaml example, then add or update coverage if the repository provides tests. Done means valid .yml files are loaded in all three RBAC locations without changing .yaml behavior.