The way you describe it: No, that's not possible.
But this is how I read the question:
A plugin (client-side) that can edit documents (server-side).
It is possible to do this, but only on the server.
That's where authentication takes place and is decided who can do what (upload/download among other things). In short: the server decides which client(s) (user) is allowed to do certain tasks.
The plugin you ask for would be a great tool for hackers