That page does not specifically address the "zip attachment" form at all, and to the extent that it does mention .ZIP extensions it (_quite_ incorrectly) implies that the virus' executable is simply packaged with such an extension. In fact, if it sends itself with a .ZIP extension, Mydoom sends itself as a proper zip archive that contains a "stored" (i.e. not compressed) copy of its executable.
Two of the copies I've gotten have been proper .zip archives (with .zip extension) which contained a UPX compressed executable, many of whose ASCII strings were further obfuscated with ROT-13.
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