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More than likely, you may need to use a server-side language for this.
I believe that fileSize is a proprietary Microsoft addition.
Are you sure Moz is loading the image? (Afraid I don't have a copy of Moz so can't try it out)
Just for debug purposes, try adding an image tag to your page and setting the source of that to your input file. If that works OK add another button and see if you can get the h/w from the <img> tag. If this works then maybe the solution for Moz is to add a hidden img tag and use that to check the image dimensions. If it doesn't work, you will at least know which part doesn't work (loading the image or getting the size).