When inventing a new file type, it is not a Good Idea to pick a number at random, because of the Birthday Paradox. If you pick a 32-bit number at random, there are over 2^32 =~ 4x10^9 possibilities, but you expect a collision to occur when just the square root of the space size is reached. This is likely to occur well before 80000 possibilities are chosen -- a far cry from the original space size!