Set Content-Type to application/octet-stream in storeReadHandler
to prevent Go's content sniffing from serving HTML payloads as
text/html. Add X-Content-Type-Options: nosniff to prevent browsers
from overriding Content-Type via MIME sniffing, and
Content-Security-Policy: default-src 'none' to block script
execution as defense-in-depth.
Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>