epa-file-name-regexp

epa-file-name-regexp is a variable defined in `epa-hook.el'.
Its value is

"\\.gpg\\(~\\|\\.~[0-9]+~\\)?\\'"

Documentation:
Regexp which matches filenames to be encrypted with GnuPG.

If you set this outside Custom while epa-file is already enabled, you
have to call `epa-file-name-regexp-update' after setting it to
properly update file-name-handler-alist. Setting this through Custom
does that automatically.

You can customize this variable.