neovim/runtime/filetype.lua

if vim.g.did_load_filetypes then
  return
end
vim.g.did_load_filetypes = 1

vim.api.nvim_create_augroup('filetypedetect', { clear = false })

vim.api.nvim_create_autocmd({ 'BufRead', 'BufNewFile', 'StdinReadPost' }, {
  group = 'filetypedetect',
  callback = function(args)
    if not vim.api.nvim_buf_is_valid(args.buf) then
      return
    end
    local ft, on_detect = vim.filetype.match({
      -- The unexpanded file name is needed here. #27914
      -- Neither args.file nor args.match are guaranteed to be unexpanded.
      filename = vim.fn.bufname(args.buf),
      buf = args.buf,
    })
    if not ft then
      -- Generic configuration file used as fallback
      ft = require('vim.filetype.detect').conf(args.file, args.buf)
      if ft then
        vim._with({ buf = args.buf }, function()
          vim.api.nvim_cmd({ cmd = 'setf', args = { 'FALLBACK', ft } }, {})
        end)
      end
    else
      -- on_detect is called before setting the filetype so that it can set any buffer local
      -- variables that may be used the filetype's ftplugin
      if on_detect then
        on_detect(args.buf)
      end

      vim._with({ buf = args.buf }, function()
        vim.api.nvim_cmd({ cmd = 'setf', args = { ft } }, {})
      end)
    end
  end,
})

-- Set up the autocmd for user scripts.vim
vim.api.nvim_create_autocmd({ 'BufRead', 'BufNewFile' }, {
  group = 'filetypedetect',
  command = "if !did_filetype() && expand('<amatch>') !~ g:ft_ignore_pat | runtime! scripts.vim | endif",
})

vim.api.nvim_create_autocmd('StdinReadPost', {
  group = 'filetypedetect',
  command = 'if !did_filetype() | runtime! scripts.vim | endif',
})

if not vim.g.ft_ignore_pat then
  vim.g.ft_ignore_pat = '\\.\\(Z\\|gz\\|bz2\\|zip\\|tgz\\)$'
end

-- These *must* be sourced after the autocommands above are created
vim.cmd([[
  augroup filetypedetect
  runtime! ftdetect/*.{vim,lua}
  augroup END
]])