My guess is that you’re reading this post because you’re wondering whether choosing one of the above naming conventions over the other will affect your work.
To be honest, this is a matter of personal opinion, which is why I categorized this as opinion.
What would I choose? I’ll name my file as YAML.
Why?
Because there was a time when we had this argument about htm/html, and today, there’s seldom a page that still has the .htm extension.
Go with .yaml.
The rest of the world, who choose .yml because their code might break on older systems otherwise, or because it’s 25% faster, will eventually catch up.
However, I must point out, at this very last minute, that it doesn’t matter which you use; what matters more, is whether your code is properly structured. Is YAML Yet Another Markup Language, or YAML Ain’t Markup Language?
That’s topic for another post!.