File storage should already work this way :). Check how the permissions are set in your system. One problem is that the legacy file-storage 5.0 did not use create privilege, only write, so there was no way to tell the difference between create new objects or edit existing object.
I thought I had changed it to use "create" in the 5.1 version, but I don't know if its practical to change the pemissions on existing objects.
