It is clear how will look situation when I will have 100% guarantee that my uploaded file will be there one year later?
Initially we cannot promise that anything you upload will still be there in a year, especially for the early life of the network we can't be sure that the same hosts will be around for a year or more. We can however provide 'health' indications of your file and repair tools - if the 'health' of the file goes down it's pretty easy to bring it back up, but you need to click a button; it's not automatic. Eventually, I do think a 100% guarantee for 1 year will be reasonable. (but not immediately!)
And will exist situation where I will not be sure that the file will be there?
For example person will upload passwords list and lose local copy.
Files will be encrypted, with the keys stored on your disk. If you lose the keys, you won't be able to recover the file. It may be possible to have keys that are like brainwallets, so all you need to do is remember a password, but for the time being you'll have to keep the keys backed up.
====
Overall, I believe that storage on Sia will be more reliable than any other solution. But until we've got at least 20 different people constantly online as hosts, I wouldn't trust file uptime at all. We won't make any guarantees, especially while the network is young.