fileupdate is kind of useless nowadays, it was more so back when you did File I/O via Scripts and the files didn't appear in the File Browser until you did /refreshfilelist.
I don't think there's a command to actually force a re-download on the client-side. You can do this though on the server-side to have everyone re-download it.
PHP Code:
function onCreated() {
forceReDownload("levels/images/block.png");
}
function forceReDownload(file) {
temp.img.loadstring(file);
temp.img.savestring(file, 0);
}