Riard suggests opting for a test network such as testnet3, testnet4, or signet over the mainnet for file sharing applications. He reasons that test networks are more suitable because the coins on these networks hold no real value, which discourages financial transactions and makes the network primarily useful for data transfer purposes. Specifically, Riard mentions that testnet4 would be an ideal choice due to its reward structure, offering 50 tBTC per block. This reward could potentially allow for up to 5 GB of data (considered as spam) to be pushed per block when utilizing a cost of 1 satoshi per virtual byte (sat/vB). This suggestion underlines the strategic consideration of using blockchain networks with lower economic stakes for applications focused on data distribution, taking advantage of the inherent characteristics of testnets to facilitate large-scale file sharing without the financial implications associated with mainnet operations.