Variable: magic-fallback-mode-alist

Like `magic-mode-alist' but has lower priority than `auto-mode-alist'.
Each element looks like (REGEXP . FUNCTION) or (MATCH-FUNCTION . FUNCTION).
After visiting a file, if REGEXP matches the text at the beginning of the
buffer, or calling MATCH-FUNCTION returns non-nil, `normal-mode' will
call FUNCTION, provided that `magic-mode-alist' and `auto-mode-alist'
have not specified a mode for this file.

If FUNCTION is nil, then it is not called.