Excellent job, Roeland!

The only suggestion is during the verification - having to select 2 files at once is a bit fiddly (on win you need to have them in the same folder I think). It would be more user-friendly I think if at verification time one could select the two files separately

Also, you could sell file storage payable in delegated SP - for instance: someone who wants to store a 500KB file could pay to have it stored with a delegation to @proof.ink of, let's say, 100SP (perhaps, precise economic calculation pending). Then @proof.ink would use that delegation to ... something that would get it some steem to pay to ... whoever hosts that 500KB file on their hard drive!