There is no docs, here is the code.

@fisker Thanks – that was some of what I was looking for.

For posterity: inferParser checks filenames (and falls back to detecting shebangs). The default language-to-parser associations are created in individual src/language-*/languages-evaluate.js in createLanguages calls. E.g.

The languages are vscodeLanguageIds. The filename-to-language associations are from https://github.com/ikatyang-collab/linguist-languages. It's actually even a little more convoluted than this, as seen in the example.