auto-mode-case-fold is a variable defined in `files.el'.
Its value is

Non-nil means to try second pass through `auto-mode-alist'.
This means that if the first case-sensitive search through the alist fails
to find a matching major mode, a second case-insensitive search is made.
On systems with case-insensitive file names, this variable is ignored,
since only a single case-insensitive search through the alist is made.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.