DownloadBook populated fileSha256 only for the kepub and pdf format
branches, so the default EPUB download never emitted the
X-Bookhoard-SHA256 response header - the hash was only available in
the feed metadata, not on the download response itself.
Populate it from mediaItem.FileSha256 in the default branch so every
download response carries the canonical primary-format hash. Clients
that capture response headers at download time now learn the hash
regardless of which format they requested.
john-okeefe
48af5d3e14