1) for file upload to function the forms encoding type must be set to multipart/form-data
From the tag selector, in Dreamweaver, select the Form tag. In the property Inspector, change the encoding type to multipart/form-data.
2) You have named the file field and the hidden form element with the same name, your form elements must have unique names, You will need to rename the hidden form element.
3) Bind the Default File option to the hidden form element after renaming it.