Some claim "Mpsum" stands for "Map Summation" or a port of a map from Max Payne or GTA IV into V. A genuine .rpf file containing custom map geometry is rare and valuable. Most links claiming to be "Mpsum map DLC" are usually just re-packed versions of existing free maps (like "Los Santos V" or "Vice City Crysis") with a new name to avoid DMCA takedowns.
Let’s ignore the malware for a moment and discuss the modding ethics. Mpsum Dlc.rpf Download