The bold text on the beta page on m0n0.ch explains it:
consolidated net45xx, net48xx and wrap images into a single "embedded" image. If you're running m0n0wall on a Soekris or PC Engines board, download the "embedded" image and rename it to reflect your current platform to upgrade via the webGUI (i.e. replace "embedded" in the filename by "net45xx", "net48xx" or "wrap")