For what it's worth, I have a DCS-2230 that's doing the same thing to me via e-mail. I do have the delay before re-trigger set to 1 second, which should be adequate. For every e-mail that contains snapshots, I get five that have one-line .log files attached. Pretty irritating. I'm using firmware 1.20.
I was able to block the .log files in my mail client by using Apple Mail's rules feature, where I can delete any messages whose attachments have .log in the name. No idea why I'm getting these in the first place, though, and I can't see in the configuration text where it might be set up.