You give Sia a budget and it does a pay-as-you-go with the budget. You can set how long you want the contracts to last, by default it's 3 months. That means you budget for 3 months of storage, and then you don't need to pay into Sia again for 3 months.

Anything that you paid for but didn't use will be refunded in full at the end of the 3 months.

Right now Sia does not support filesharing of any kind, however that's in the mid-term (6-12 months) roadmap.