Upload a file to LinShare using its rest api.
        The uploaded document uuid will be returned

def update_file(self, uuid, file_path, async=False):
        """ Upload a file to LinShare using its rest api.
        The uploaded document uuid will be returned"""
        url = "documents/{u}/upload?async={a}".format(u=uuid, a=async)
        return self.core.upload(file_path, url, http_method='PUT')