did you name it update.zip, or just update? you may have named it update.zip.zip
a way to tell is go to control panel>appearence>folder options>view then make to uncheck "Hide extensions for known file types"
go back to your update.zip, and make sure it says that and not update.zip.zip
edit - this is just an idea, I'm not sure why if that's not it.
Pretty sure its just update.zip. when I look at it in astro it shows just update. Can someone provide me with a known link that works