Well, I've tried a few other php email attachment applications, and have been getting the exact same error or something similiar. Since I've already setup a php guest book that works fine, it leads me to believe there is probably an issue with the servers php .ini file that may limiting this sort of transaction. I do have a regular contact page that doesn't require an attachment field, so maybe I'll learn more when I go to set it up. Though I can't promise the ultimate form will be up since I'm circulating through a few differnt ones.
If you get bored, you can take a quick look at http://www.handyblues.org.
Thanks!
sidmel