Here is a file you can use to test if uploading files works on your server.
Just browse to this page and try uploading a file. See if you get a success or failure message after uploading the same file you are trying to attach to your email.
If you get a success message, please send me a zip of the form page you are trying to use to send the email as well as the email configuration page that will be named after the page you applied Universal email to. It will be in the webassist/email/ directory.