The file is a WIP version of my Killing Floor mod MUTATOR.
Here is a complete version of the KFMod20 MUTATOR, but is still work in progress. It lets you get the new PERK, ChemBug and lets you acquire the new weapon, ChemicalThrower to use against the specimens.
So, who to use it? I have installed it on KF v 2.52, and see no changes, except for some bots having this perk icon, and th Handcannon texture missing or invalid (the weapon now has no texture at all, and is all grey like if it's made of concrete) and my Support perk icon has some blue, green and yellow marks, which look like some other perk's icon mixed into it.