How to upload files for use in built game from user's computer?

Hi guys,

So I am trying to create a game where users can upload files such as images and text files for use in the game to create materials, color the terrain, etc. The catch is that I am going to assume that they do not have Unity Editor and that all they have is the built game. Is there a way to do this? I am aware of Asset Bundles but that requires going through a server which I don't think would work since they would still need access to a computer that has Unity Editor on it if I understand them right.

Any help is appreciated guys, thanks!