It is a diskboot.bin and the stuffit expander says "The StuffIt Engine was unable to determine the file format." I downloaded the correct firmware from Might-hoernsche.de. I am using a MAC.
You need a file named "ps.fi2" in the root of your SD card to make the "firmware update" starting method work. Please extract the whole archive content to the card, including the CHDK subfolder.
Note that it may matter which mode is selected on the camera's mode dial. "Simple" mode for example won't show the "firmware update" menu entry.