Yaml files can be created with yaml and yml file extension, Both are same in interpretation and syntax.

In Older versions of windows, Extensions are restricted to 3 letters like .yml

Now a days, There is no OS system level enforce to have 3 letters in extensions.

Most of the yaml users are using .yaml as preferred choice

Conclusion

As long as content inside yaml and yml follows the yaml.org syntax, There is no value in extension of yaml or yml. Both performs same in syntax and programming language interpretation