The words or term “real estate mortgage” as used in this article shall be understood to include every species of conveyance intended to secure the payment of money by lien upon real estate. Any contract for the sale of real estate in which title is retained in the vendor for the purpose of enforcing payment of the balance due shall be deemed a mortgage upon real property. If an indebtedness is secured by both real and personal property, said mortgage shall be deemed to be a mortgage on real property for the purpose of this article. Any contract or agreement by which the indebtedness secured by any mortgage is increased or added to shall be deemed a mortgage of real property for the purposes of this article and shall be taxable as such upon the amount of such increase or addition.