All individual or fiduciary income tax returns required by law to be filed with the commissioner of the revenue shall be filed with the commissioner of the revenue within the time required by this chapter, unless the time for filing such returns is extended by the Department. Upon all returns on which tax is due, filed with or assessed by the commissioner of the revenue after the time herein prescribed for the filing of returns, the commissioner of the revenue shall assess a penalty equal to six percent of the amount of taxes assessable thereon if the failure is for not more than one month, with an additional six percent for each additional month or fraction thereof during which such failure to file continues, not exceeding thirty percent in the aggregate. Such penalty shall be collected in the same manner as is provided by law for the collection of other taxes.