magic-mode-alist

magic-mode-alist is a variable defined in `files.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Alist of buffer beginnings vs. corresponding major mode functions.
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 rather than allowing `auto-mode-alist' to decide the buffer's
major mode.

If FUNCTION is nil, then it is not called. (That is a way of saying
"allow `auto-mode-alist' to decide for these files.")