def METHOD_NAME(params):
    # Called if there is no policy supplied by the user.
    # Creates a policy that provides access for 24 hours.
    delta = datetime.timedelta(hours=24)
    expires = datetime.datetime.utcnow() + delta
    expires_iso = expires.strftime("%Y-%m-%dT%H:%M:%S.%fZ")
    policy = POLICY.format(expires=expires_iso,
                           bucket=params['Bucket'],
                           prefix=params['Prefix'])
    params['UploadPolicy'] = policy