I know this is a 3+ year old thread but I still haven't been able to get this to work correctly with Dropbox (or several other apps that expose files through the External Files API).
I have confirmed that overall the process works as I've been able to get Working Copy to show up as expected as a file provider when I click Pythonista > External Files > Open... However Dropbox always seems to show up disabled and I am unable to browse it to select files.
Working Copy and Dropbox are both configured as external file providers in the Files app, and I'm able to browse their contents from the Files app. However when trying to browse them from Pythonista, Dropbox remains disabled.
Is there any other configuration required to fully expose a 3rd party file provider in Pythonista?