Given a GridFSBucket implementation when calling the OpenDownloadStreamAsync method a GridFSDownloadStream is returned.
The stream returned from the API does not allow us to copy it. When using the CopyToAsync method, a NotImplementedException is thrown, and a NotSupportedException is thrown when using the CopyTo and Read methods. ReadAsync seems to be the only method that works correctly.
I have attached a .NET sample to show the scenario.