When I upload an image file to a blob, the image is uploaded apparently successfully (no errors). When I go to cloud storage studio, the file is there, but with a size of 0
The Position property of the InputStream of the HttpPostedFileBase had the same value as the Length property (probably because I had another file previous to this one - stupid I think!).
All I had to do was to set the Position property back to 0 (zero)!