extend stream-mechanism for "get filesize"

[ptahmose]

Is your feature request related to a problem? Please describe.
It would be beneficial if an input-stream could provide the file-size. C.f. here..

Describe the solution you'd like
The stream-objects (created by CreateStream) should provide a way to retrieve the filesize (if supported).
Maybe introduce an interface e.g. IStreamEx (deriving from IStream) which contains this functionality.

Describe alternatives you've considered

Additional context
N/A