I just had the same problem and tried quite some time to fix this. Couldn't see the SD card option under Files. In my case the problem was the following. I formatted the SD card as Internal storage but didn't migrate the data. In Android setting do this:
Settings > Storage > Internal -> on the right corner press Migrate data.
After that I could access the SD card over the web interface. Maybe it works for someone else as well.
Settings > Storage > Internal -> on the right corner press Migrate data.
After that I could access the SD card over the web interface. Maybe it works for someone else as well.