With AltaMail you can transfer files, videos, photos and documents between your device and different cloud servers

When viewing a files screen, either local files or cloud server files, tap on the "+" button in the top right and select "Import". This will allow you to choose another location to select files and import them into the current file screen.

You can also select one or more files in a Files screen and then choose "Export" from the action menu (circle with 3 dots button).