On the client side, you won't be able to manipulate the filesystem at will unless you have some help of an extension of some sort (e.g. NPAPI based).
Futhermore, you mention from server to client: I hope you are not implying starting a connection on the server side to a client browser because in this case this is clearly not possible.
Of course, if you are talking outside of the browser environment (e.g. using Javascript (Rhino)) on a Desktop application, then this changes the situation (and capabilities).