Android Question How do I set my app's photos/files to be backed up by Google Photos/Backup

have you tried this: https://www.b4x.com/android/forum/threads/add-image-to-gallery-android-5-10.121992/#content ?

Google Photos will not backup your images if they aren't considered to be in the gallery.

There is a built-in backup feature which is enabled, unless you explicitly set android:allowBackup to False in the manifest editor.
It should backup your app files automatically. Though I don't think that you have any control over its exact behavior.