The application for a new trial shall be by motion, upon written grounds, filed at the time of making the motion. It shall be sufficient, however, in assigning the grounds of the motion to assign the same in the language of the statute and without further or other particularity. The causes enumerated in subdivisions (2), (3), and (7) of section 25-1142 shall be sustained by affidavits showing their truth and may be controverted by affidavits.