According to yaml.org, the official file extension is .yaml.
Quote:
Is there an official extension for YAML files?
Please use “.yaml” when possible.
However there seems to be a disagreement on the internet on which extension to use. If you look up examples on the web, many of them use the unsanctioned .yml extension.
Searching Google returns nearly 3 times as many results for the shorter one.
49,100
15,400
So which am I supposed to use? The proper 4 letter extension suggested by the creator, or the 3 letter extension found in the wild west of the internet?