Is there an app for Android and/or Windows that could transfer files to and from Mi Box S via WLAN?
Preferably there would be no interaction needed on Mi Box S. Just choose the files and Android or PC.
I was thinking to use it as a wireless backup server for my photos and videos,...
there are several apps on play store you can try. smbtv, ftp server, syncthing, sendfiletotv, airexchange, ... pick whichever suit you best.
I think it’s possible with ES File Explorer.
I use the Bubble UPnP app with paid upgrade to do this with an old Android box and a HDD connected a a backup drive. I set it up just to repurpose the old box and an old drive, and didn't think it would work as well as it does. The download speed has been relatively good/fast, but noticably slower when I have tried using Bubble to download a video from another network device to our Nvidia Shield TV, or vice versa. Not sure if something @ Android TV or Nvidia Shield is what causes it to take so much longer. You can try the free version of Bubble though to test it. If it works for you, you may not need the paid upgrade either.
Set up File Server with MIXplorer
I think you would be better suited to a NAS