The last time I used Yahoo IM, the file transfer service was indirect. It would upload to a Yahoo server completely before the recipient could start downloading it. And, the download was through http.

However, I would imagine that there is a file size limit due to it being stored first. Also, this was in the early days of Yahoo IM, so things may have changed.

Also, AIM transfers usually only fail if both people are behind NAT/firewall. I am using NAT and I can always transfer files to my 56K friends (but why would you want to).