I just recently installed Ubuntu MATE 20.10 on my RasPi 4B. I wanted to install caja-dropbox but don't see it in the Software Boutique and I tried installing it from the command line:
actually it does support more than just ext4, however it states clearly on the dropbox website that it does not support arm processors on linux, so my guess is as dropbox does not work which caja dropbox depends upon, it is not even available to install on arm architectures