`upload_files` doesn't work when you send a list of paths to it

[kbolashev]

Discovered by Jinen

We need to make sure it's usable by users without having them figure out what the list actually has to be, which is a tuple of (path, binary_data). Right now the workaround is to basically map(lambda p: DataSet.get_file(...), paths). Maybe that will be enough to handle most cases.