The problem seems to be either low-memory or i386 related. I tested on an amd64 machine with 3GB RAM, and the problem goes away with 2+ GB files.
The initial machine is i386 with 512MB RAM. 1GB files work fine, but when they get stepped up to somewhere around 2GB, I start seeing the negative Content-Length. I'm not sure if this well help, but I'm using this as an excuse to upgrade the problem machine to amd64. I'll debug further or report back if the issue goes away.
