You either need to upload the image before you are sent to this form page or you can have the upload occur after the form is submitted but before the record is inserted.
It is not possible with DataAssist to have the file upload before the form is submitted. Are you wanting to have your insert occur only after the file has been successfully uploaded? If so you can apply the DFP upload server behavior then in your DataAssist insert server behavior you can update the trigger to be files successfully uploaded.