First open file 1, then fly to any Owl save block but not by Milk Road. Then put on the mask you what to use on Epona save at the Owl and open file 2. Ride Epona to Milk Road and save. Open file 1 and there you are on Epona.

NOTE: Don't use Fierce Diety trick then this one. Because the sword he carrys is an auto equipped weapon so if you do that you will be stuck in the bow screen. Also don't press B button when your on Epona as a Link form.