It is not possible to dynamically change the value of a file field, otherwise you could set it to "c:yourfile" and steal files very easily.
However there are many solutions to a multi-upload system. I'm guessing that you're wanting to have a multi-select open dialog.
Perhaps have a look at http://www.plupload.com/ - it's a very flexible solution to multiple file uploads, and supports drop zones e.t.c.
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