On older browsers you could get hold on the file path on the client machine. On modern browsers, you can't get this info (except maybe for trusted sites in IE).
Giving too much info about the directory structure of the client machine is considered a security issue. You can not access it with javascript, and it is not posted to the server.
For further reference, have a look at : Is this possible to get full client path in <asp:FileUpload> button?