Opening an IStorage from an IStream

I'm implementing a property handler for a structured storage file and would like to initialize it with IInitializeWithStream given its benefits of isolation, handling slow transfer, etc. But I see no obvious way to open an IStorage from an IStream. I don't want to load the whole file into global mem, and the documentation doesn't state whether ILockBytes is necessarily implemented on the IStream passed to Initialize.

Imple开发者_如何学运维menting IInitializeWithFile would be easy, but then cannot be isolated.

Any thoughts to how I might be able to get an IStorage from this IStream?

To note, I do not own this file type but for my common work having these extra properties exposed would be helpful.