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Magnetic compounds with significant and temperature-insensitive magnetoresistance are quite promising for device applications. Here, we intentionally doped different amounts (0.025-0.1) of Co into epitaxial (La0.7Sr0.3)MnO3 thin films. The evolution of electrical, magnetic, magneto-transport, and valence state of this series of Co-doped (La0.7Sr0.3)MnO3 have been systematically characterized. Remarkably, the magnetoresistance in 0.1Co-doped (La0.7Sr0.3)MnO3 can reach significantly 35% within an extremely large temperature window (from 40 to 200 K). Other Co-doped epitaxial (La0.7Sr0.3)MnO3 thin films show similar sharp MR-peaks around Curie temperature as that in (La0.7Sr0.3)MnO3. The evolution of magnetic and magneto-transport properties of Co-(La0.7Sr0.3)MnO3 films can be explained by antiferromagnetic coupling induced by paramagnetic/low-spin Co3+(Co2+) ions in (La0.7Sr0.3)MnO3. Our work demonstrates that doping paramagnetic/low-spin Co into manganites is an effective way to optimize their magneto-transport properties.