With a dremel cutting disc, cut a notch across the screwhead for a broad flathead screwdriver and you should be able to get it out.
Alternatively, looks like you can access the bottom end of the screw and add a notch at that end for a smaller screwdriver.
BUT first I'd try and get a broken screw extractor. Won't be the only time you need one. Get at M10, bunnings, Hammers, Tool shed etc.
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