AFAIU the list holding the values is called auto-mode-alist
constructed as pairs of suffix-and-mode-to-invoke
look into, which suffixes trigger nxml-mode
to insert for every suffix at stake the new one write into your init:
(push '("\\.MY_ENDING$" . xml-mode ) auto-mode-alist)
maybe it's needed still to remove the old entries.
HTH,
Andreas