A high power mid-infrared (MIR) supercontinuum (SC) is demonstrated in a strictly all-fiberized Er-doped ZBLAN fiber amplifier (EDZFA). The EDZFA is seeded by a 2.0-3.5 µm fiber-based SC laser and pumped at 976 nm. At pulse repetition rate of 500 kHz, the output SC spanning from 2.7 to 4.2 µm reaches a record output power of 4.96 W with an overall slope efficiency of 17.3%. This Letter, to the best of our knowledge, demonstrates the first all-fiberized in-amplifier SC generation in the MIR region, which has significant potential for further power scaling.