this not really is a solution, because usually apps, which use that permission are rejected by Google, see here, there are a few exceptions...
we still want to be able to access files, which have been provided by other apps in the shared directories... it looks like it is not possible anymore in App Inventor now starting with the new SDK30 release? However theoretically it is possible to still access these files.... see here Cómo abrir archivos con el framework de acceso al almacenamiento | Desarrolladores de Android | Android Developers @ewpatton, do you plan to add this back into App Inventor?
When should you request the All files access permission?
You should only access the all files access permission when your app cannot effectively make use of the more privacy friendly best practices, such as using the Storage Access Framework or the Media Store API
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.