Again all of this assumes unifrom distribution. The files used in dropbox are not guaranteed to be random or uniformly distributed. As I already said in my initial comment, various different files types have internal structures which may (again if you are unlucky) result in multiple different files of those types tending to have similar hashes.
One of the goals of a good hash is a uniform distribution. The source files don't have to be random at all. I wouldn't put any money on the odds of by accident hitting a systematic flaw that security researchers haven't yet found in analyzing the hash.
That said, I think it will be possible to deliberately create a collision in SHA256 at some point in the future.
