As Steve Blank says, "most products fail for lack of customers, not technology." Why would you spend time building a product--almost any product--unless you are absolutely sure there's a market for it? What makes your file sharing service different from the others? Where will its market share(and revenue) come from? Why will people choose your product instead of entrenched competition? And why would you spend weeks or months building something unless you're absolutely certain there's a market for it?