Comment 2 for bug 1675276

It looks like cinder's os-attach API does require the mountpoint parameter:

https://github.com/openstack/cinder/blob/504f129714c60eb2d24192462113db971a798b1a/cinder/api/contrib/volume_actions.py#L62

In fact, it looks like a request would fail with a 500 error if the mountpoint is not provided since it doesn't check and would raise a KeyError if not provided.