It's not the content length, because other download programs get the size just
well.

<http://www.10-david.com/dl.asp?id=350>

One example, I downloaded Swat3 missions and mods from there. The actual file
comes from 10-david.wargamer.net/download .

Using +*dl.asp* for those links and +*.zip* for the actual resulting zip files
from those links. It actually took me a while to figure out that the resulting
links need their own passing scan rule, just having the "get external files"
option on was not enough. Perhaps you could elaborate on this behavior more in
some of the help files etc, it is not clear at least to me how the engine
behaves in situations like this.