How are you running RStudio Server? I'm not sure how you do it natively with RStudio Server, but it seems like you could try file.choose to see if that allows them to get into the folder.
If you are running it as a container with docker, you can map your local network drives to make them available to the container session like this:
docker run -d -p 8787:8787 -v ~/Desktop:/home/rstudio pgensler/sandboxr
which maps the Desktop folder (volume) to /home/rstudio from the image pgenlser\sandboxr located here. Does that help?