On multipage forms with a file upload, I was constantly getting the above referenced error. The single page forms with upload worked fine.
This was a nagging problem that was discovered only after completing a four page form. I searched relentlessly within this forum to no avail. Finally, I found the solution on another php site/forum. Hopefully, this will help others.
1. - On the multipage forms, I had to ensure that formmail was in the same directory as the multipage form itself.
2. - Scratchpad needed to be set, which is information included in the existing documentation for formmail.php.
3. - the user name provided by my server/host had to referenced in the Scratchpad setting, rather than the web address itself.
"/home/your-site/fmscratchpad/" (did not work for me).
In my case, Item 2 was already done. Items 1 & 3 were done after google searching. This may not be a total solution for everyone, but after my experience, be assured that it is a good place to start for similar problems with upload files for multipage forms. Let me also state that this worked for me - I certainly can not claim it as a rule.
Finally, thanks Ginger23 for assisting me with previous implementation challenges. I belive formmail.php is a great product, I just wish the documentation was more precise.