The app has 3 ways to upload new pictures, one of them is the "custom picker".
The custom picker has an activity that shows a list of folders:

The present issue is about that custom picker's folders list activity.
The idea is to implement long-press on a folder to reveal a menu offering folder operations:
Phase 1:
- Open the folder in another app (just launch an intent, Android should hopefully suggest available file explorers and gallery apps).
Phase 2:
- Mark all pictures currently in this folder as "Not for upload".
Phase 3:
- Always ignore this folder (which makes it greyed out, with 🚫 overlay icon, shown at the end next time the picker is opened)
- Un-ignore if the folder if it is the "Ignored" state.