@cvp thank you for your hint. But unfortunately your referenced file picker is not the standard ios file picker. Therefore you can only pick files within the pythonista app itself.
The one in the dialogs package brings up the iOS system wide file picker and it lets you browse throughout all your apps of your iOS-Device even including your iCloud documents.
But I can't select some of them. In fact I don't know how to handle the types parameter from dialogs.pick_document() ... The internal pythonista documentation says
dialogs.pick_document(types=['public.data'])
Show the system’s document picker for importing a file, and return the file’s path (or None if the dialog is canceled).
types specifies the Universal Type Identifiers (UTIs) that should be selectable in the document picker.
The default (['public.data']) makes all regular files selectable.
A list of the system-defined UTIs are available in Apple’s Uniform Type Identifiers Reference.
The return value is a temporary file. You can read it directly, but to keep a permanent copy, you must move it somewhere else.
Apple has this documentation declaring new type identifiers. But its hard for me to get the clue how that correspondents to the type parameter of dialogs.pick_document(...)