How to add additional filetypes for LSP clients?

[rolfb]

What's the conventional way of adding json to filetypes which lsp clients for tsserver or eslint will attach to with this lua-based neovim setup provided in this repository?

 2 active client(s) not attached to this buffer: 

 Client: tsserver (id: 2, pid: 43699, bufnr: [9])
 	filetypes:       javascript, javascriptreact, javascript.jsx, typescript, typescriptreact, typescript.tsx
 	autostart:       true
 	root directory:  [hidden]
 	cmd:             typescript-language-server --stdio
 
 Client: eslint (id: 3, pid: 43700, bufnr: [9])
 	filetypes:       javascript, javascriptreact, javascript.jsx, typescript, typescriptreact, typescript.tsx, vue
 	autostart:       true
 	root directory:  [hidden]
 	cmd:             vscode-eslint-language-server --stdio