How can a plugin override the language for a specific filename? #16621

Yes, filenames is the right mechanism, and it does beat extension inference. When Prettier infers a language for a file it matches in two passes: first it checks every language's filenames for an exact basename match, and only if nothing matched does it fall back to matching extensions. So a plugin language with filenames: ["something.js"] wins over the built-in JavaScript language for a file named something.js, even though .js would otherwise match babel. Plugin languages also take priority over the core ones (the search list is built with the plugins reversed, so later-loaded plugins win over earlier ones and over the defaults).

A few things to check if it's not kicking in:

• The match is against the file's basename, exactly. filenames: ["something.js"] matches src/something.js but not something.test.js — no globs.
• Your parser needs to be minimally complete (parse, astFormat, locStart, locEnd) and the printer registered under that astFormat. If the parser throws, it can look like the language was never picked.
• You can verify what Prettier decided with prettier --file-info something.js --plugin ./your-plugin.js — it prints the inferred parser without formatting anything.

Also worth knowing: on newer Prettier versions a language can define an isSupported({ filepath }) callback, which is checked after filenames/extensions and lets you match arbitrary paths when a fixed filename list isn't flexible enough.