To help you correctly, I need to know which you are referring to, as this generic name is often used for several different things:
A data file for a particular program that uses "unknown" as a placeholder prefix for its resource packages. Are you trying to open/extract this specific file, or unknown.part06.rar
A specific piece of a split data set (like a game, software, or media collection) where the contents are hidden until all parts are combined. To help you correctly, I need to know
This file name suggests you are dealing with a , specifically the sixth volume of a larger set. To help you correctly
A file from a website or peer-to-peer network that hasn't finished downloading or has lost its original name.