auto-coding-regexp-alist is a variable defined in `mule.el'.
Its value is

(("\\`BABYL OPTIONS:[ ]*-\\*-[ ]*rmail[ ]*-\\*-" . no-conversion) ("\\`\376\377" . utf-16be-with-signature) ("\\`\377\376" . utf-16le-with-signature) ("\\`\357\273\277" . utf-8-with-signature) ("\\`;ELC" . emacs-mule))

Alist of patterns vs corresponding coding systems.
Each element looks like (REGEXP . CODING-SYSTEM).
A file whose first bytes match REGEXP is decoded by CODING-SYSTEM on reading.

The settings in this alist take priority over `coding:' tags
in the file (see the function `set-auto-coding')
and the contents of `file-coding-system-alist'.

You can customize this variable.