Manifest files with `yaml` extension are ignored.

[johrstrom]

Describe the bug

Manifest files with yaml extension are ignored.

To Reproduce

Create an app with manifest.yaml instead of the yml extension.

Expected behavior

The app behaves as it should.

Environment:
Any

[johrstrom]

Note this is from discourse: https://discourse.openondemand.org/t/published-passenger-app-doesnt-appear-in-dropdown-menus/4901/8