Yeah, I was confused. I'm a product guy, so math isn't my strongest point. It looks like combining Md5 + size + Sha1 should be enough to get a almost unique file id. Thank you all for the replies.
The funny thing is that the blog's author himself succeeded without talking to customers. Imagine if he talked, found out nobody wants it and quit without pivoting.
Can you quote how many hours Google founders spent talking to customers?
How many Twitter founders spent and why they still don't know the business model that will work?
Facebook started as somewhat of a ripp-off but actually people weren't willing to 'buy' it, hence these stories over IP that Facebook got into over the initial days