Just wanted to let you know I have discovered a bug in 1.4 Beta 4.
To re-create you need to setup a form that contains a "custom field" form field and set its "field type" to "file upload". Once you have done that make it a "required field" by checking the check box.
The form will not post with this field marked as required. Uncheck it and the upload goes through perfectly.
Great new feature to be abled to upload the path to custom fields but I am guessing its not checking for something correctly.
Hope that helps