org-babel-tangle-lang-exts

org-babel-tangle-lang-exts is a variable defined in `ob-tangle.el'.
Its value is

(("emacs-lisp" . "el"))

Documentation:
Alist mapping languages to their file extensions.
The key is the language name, the value is the string that should
be inserted as the extension commonly used to identify files
written in this language. If no entry is found in this list,
then the name of the language is used.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.