Currently, fetching a blob while providing a progress bar is a two-step process:

• Call Stat to verify that the blob exists and determine the size
• Call Open to get a ReadSeekCloser that provides the blob data

The HEAD request that Stat performs here seems to be redundant, since the same information could be gathered from Open's GET status code and Content-Length header. However, Open currently doesn't expose the Content-Length. Doing so could save a round trip. Perhaps this could be done by having Open also return a Descriptor, or creating another variant of Open which does, if returning the descriptor imposes some extra for certain implementations.

cc @dmp42