In my case, I wasn't looking for a default file formatter, but rather applying an existing file format to a custom file. E.g. say you have a custom file file.framework but you have developed this framework file to follow a YAML standard. Initially VS Code won't render it properly, it'll just treat it as text, but you can select the language format.

1. Open the file
2. On the bottom right corner, select the Plain Text option
3. Click Configure File Association