How to assign more than one file type in VS Code

Question

I'm working on a VS code extension which 'augments' a particular language - specifically it's for textmate grammars, which can be written in JSON format. I want to enable the extension for .tmlanguage.json files but without disabling VS Code's support for .json. In package.json I have

 "languages": [
      {
        "id": "json-tmlanguage",
        "extensions": [
          ".tmLanguage.json",
          ".JSON-tmLanguage"
        ],
        ...
      }, ...
]

and

 "activationEvents": [
    "onLanguage:json-tmlanguage",
    ...
  ],

This activates when a .tmlanguage.json file is opened but seems to prevent normal JSON processing (e.g. Shift-Alt-F Format Document)

I don't want the tmlanguage stuff to activate on any JSON though, just the ones with the specific extension.

Answer 1

It is not possible to assign more than one language at the same time to a single file.
Every extension you wish to use on a file, needs to support the language that's being assigned.

The built-in JSON extension recently had the PR merged.
Extension owners can now add the following code to their package.json file.
Which will enable JSON language features on their languageId.

"main": "",
"extensionDependencies": [
    "vscode.json-language-features"
],
"contributes": {
    "jsonLanguageParticipants": [
        {
            "languageId": "foobar"
        }
    ]
}

extensionDependencies activates the json-language-features extension.
main is required for extensionDependencies to work.