The recently lost file upload feature in the Nextcloud app for Android

This is also why the official SyncThing Android app stopped being distributed. There is a fork but it's not available on the Play Store.

The problem with the SyncThing Android app is that it's just a wrapper around SyncThing, which is a Go library, but SAF does not give you simple file descriptors you can use in native code. Instead, you get "content://" URLs, and you need a Java/Kotlin bridge to convert these to file descriptors. That would need to be done in SyncThing itself (EDIT: or some other trickery, because it seems like syncthing-fork made it work somehow).

However, AFAIK, this problem would not apply to the NextCloud app.

> and you need a Java/Kotlin bridge to convert these to file descriptors.

Do you need it in these languages or could you use anything that can make binder calls?

To my knowledge you cannot access SAF through binder, for sure not officially.