Return a string specifying the files to submit for this project.

def submit_string(self):
        """Return a string specifying the files to submit for this project."""
        required = []
        optional = []
        for file_verifier in self.file_verifiers:
            if file_verifier.optional:
                optional.append('[{0}]'.format(file_verifier.filename))
            else:
                required.append(file_verifier.filename)
        return ' '.join(sorted(required) + sorted(optional))