Unfortunately Samsung's "My Files" app does not recognize kma files. But Google's "Files" app will open them in Custom Maps when you tap them.
Notice that the new version keeps the maps in the app's internal storage area (Play Store requirement), so any maps created with the new version of the app will not show up in that directory. But you can export the maps out of the app and keep your map collection on Google Drive or on some other file storage where you can open them when you switch phones.
Marko Teittinen
Author of Custom Maps