.yml is simply the three-letter spelling of the YAML extension — the contents are ordinary YAML in every respect. It became entrenched because early adopters favored terse extensions, and flagship filenames like docker-compose.yml and .travis.yml cemented the habit.
Batch files can each use a different output. Nothing uploads for local conversions.
Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.The YAML project has recommended the full .yaml extension since its early days, but influential tools grew up around the shorter form regardless. Ecosystems now increasingly accept both — GitHub Actions, for instance, discovers workflow files with either spelling.
Anything that reads .yaml reads .yml identically, from language libraries to editor plugins. The only compatibility question is which spelling a given tool looks for by default when discovering config files.
