Not an answer to the question since you've already solved it, but I thought it may be useful to anyone else with similar issues. I had a recent issue where I wasn't keeping submission logs and hadn't set a root folder. No files got attached to emails.
We wanted a photo submission form that would keep the file assets on our system, we have diskspace issues. Instead the image should just be sent to the recipient and not kept of the system.
If you are keeping submission logs, the attached files will sit under the submissions (unless you define a root node, in which case they are kept there). If you are *not* keeping submission logs and have not defined a root node, then the file does *not* attach to the email. The way around this is you have to specify a root note.
To get around our disk space issue (we didn't want a buildup of large image files), you *can* set the root node to the Trash. This has worked fine for us so far since once the image is delivered to the recipient we don't want it anymore. Then our normal Trash purge process kicks in (in our case weekly) and no buildup of files.