A quantum theory of spin-dependent electronic transport through a single Mn2+-ion-doped II-VI quantum dot weakly coupled to ferromagnetic (FM) leads is presented. The electron-Mn-ion exchange interaction in the quantum dot strongly enhances the spin polarization of the current and the tunneling magnetoresistance (TMR) and yields a highly spin-polarized current even when the FM source is not 100% polarized. Both the spin polarization and the TMR depend strongly on the bias voltage VSD. In the presence of an external magnetic field, the TMR oscillates with VSD. The results open the way to voltage-controlled spin filters and magnetic sensors.