This has been helpful for me when trying to figure this out:
-
The iOS closed File System (aka sandbox)
https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html -
And this thread helped me get started:
Sandboxed ios app "Documents" folder location?
You can ofcourse set a FileBrowser to the right path and use its method for dragging/clicking to handle the files.