Last-Modified header is missing

For the Content Delivery service to cache your content,
your origin must provide the following two HTTP response headers with every object:

Last-Modified-the date and time the object was last modified on the origin server (typically the same as the file timestamp).
If the Last-Modified header is missing, and there are no additional cache-control headers (such as Expires or Cache-Control ),
Content Delivery will not cache the file.