Download the file of a kept inspection

Only when the account keeps files (bytesDays) and this inspection's file is still kept. The bytes are decrypted and authenticated in full before the first one is sent. The download is named <id>.<extension>: the original filename was never stored.

404 inspection_not_found (no such record in this account), 404 file_not_retained (the record exists but its file was not kept or was deleted), 410 file_expired, 503 retention_bytes_unavailable (the server cannot decrypt files right now), 503 busy with Retry-After (too many kept files are being served or inspected at once). A download that stops being read for 5 seconds, averages under 64 KB/s, or runs past 30 seconds is cut off.