@TestPropertySource not recognizing yaml files

Spring Boot supports not only application.properties but also application.yml files for long time now and we are using it in all our projects for better readability. But when it comes to testing with annotations like @TestPropertySource then yaml files are not supported for whatever reason. If you try to specify a yml file as source, it does not complain but the test also won't be able to resolve the properties unless you are using the properties notation format and just store it with the .yml file extension.

I wonder why this is not supported yet. Is there another annotation I haven't seen yet or is there any plan to let TestPropertySource automatically recognize the file extension and make use of the corresponding yaml file loader?

Start by tracing how @TestPropertySource loads and parses its specified files, then compare that path with Spring Boot's existing application.yml loading support. Done means a .yml or .yaml source is recognized by the annotation and its properties can be resolved in tests, with coverage for the supported YAML extensions.