Download Archive fails with flysystem

Archives from cloud storage has always been a bit tricky for Filebrowser. The code relies on realpath() which will fail in case of a cloud storage.
If have revised the code and made it possible to download archives of files on cloud storage, but realize that the files are first downloaded and then added to the archive, so this is to some costs.

In case a folder (directory) is selected, Filebrowser now will reject the archive and provide a message instead of failing. I have also updated the README.
It is added to todays .dev version and I will also upload a patch.
let me know if it works.

Patch #4 works for me. Thank you very much!