This reports the current state of the actual file transfer underlying this Downloader job -- for example downloading, downloaded, or failed. This is the field that most directly answers whether this specific firmware file is actually moving across the wire right now, distinct from the higher-level FSM status fields that describe the broader workflow around it. A transferState of 'downloaded' combined with a populated checksum on the corresponding Image object means this file has fully arrived and is ready to be referenced by a host firmware pack.