How do I add custom filetype detection for a new file extension without modifying Vim's built-in filetypes?

Explanation

Vim automatically sources every file in ftdetect/ directories on the runtimepath when filetype detection runs. Creating ~/.vim/ftdetect/mytype.vim (or ~/.config/nvim/ftdetect/mytype.vim) lets you add custom filetype associations for new extensions, patterns, or tools — without ever touching Vim's built-in filetype files.

How it works

• Vim's :filetype detect sources all ftdetect/*.vim files from every directory in runtimepath
• Each file in ftdetect/ should only set the filetype (via setfiletype or set filetype=)
• setfiletype is preferred: it respects any filetype already set by a prior rule and won't override it
• Files in ftdetect/ fire after Vim's built-in detection, so they can override defaults

Example

Detect .tf files as Terraform and .jsonc as JSON with comments:

" ~/.vim/ftdetect/terraform.vim
autocmd BufRead,BufNewFile *.tf,*.tfvars setfiletype terraform

" ~/.vim/ftdetect/jsonc.vim
autocmd BufRead,BufNewFile *.jsonc,tsconfig*.json setfiletype jsonc

Now when you open a .tf file, Vim automatically sets filetype=terraform.

Tips

• For complete filetype support, also create ~/.vim/syntax/mytype.vim for syntax highlighting and ~/.vim/ftplugin/mytype.vim for buffer-local settings (indent, commentstring, etc.)
• Filetype names must be alphanumeric with dots (e.g., c.doxygen for C+doxygen)
• Check what Vim detected with :set filetype? or :echo &filetype
• List all filetype detection files with :scriptnames after opening a file of that type