avast! didn't really miss anything or detect it "too late". There is no such thing really. However it sometimes does happen that parts of the file do fall through Web Sheild and hit the HDD where Filesystem Shiled detects it because browser already cached parts of that file. But in such cases they are just remnants of the actual file. And even if they are in fact a full file which was initially detected, there is no way of it getting run anytime later. In worst case it will just be on a HDD and will not do anything. What avast! found out later was probably just one such file inside browser cache that File shild got hands on while browser wanted to purge that file from the browser cache sometime later.