Access a file in the disc with one name and save it (save as dialog) with a different name...

Can that be done with javascript?

Nope. JS can't access files. It might be possible if you use adobe air, or some other 3rd party framework, though.

Not on the client....

In your server, you need to set the response header before sending the file.

Java code:

response.addHeader(
     "Content-Disposition" ,
     "attachment; filename=" + fileName );

If you are using another server, the header you want to add is the same