I am confused about the fees for storing files. What is to stop someone from modifying the code to just insert files for free? From what I understand there is nothing in the algorithm that would actually prevent that.
Thank you for the question. Nothing stops us from modifying mintxfee as soon as txfee isn't a hard rule: block will be accepted with zero tx fee.
There is two kinds of mintxfee:
1) minimal tx fee that limits me (as a miner) to accept tx into block
2) minimal tx fee that limits me (as a node) to relay tx to other nodes
As a miner I can accept lowfee txes into my memory pool but these txes will be accepted by other nodes only with blocks I've mined myself. This means that lowfee txes will reach other nodes much slower.