Unfortunately, DRE works on lower level than Exchange/Outlook application.
The cache is only populated if there is another device at the other side of WAN link
that has a chance to cache the same data and create saving in WAN usage if it
is transmitted again, in any direction. This happens transparently, the server does
not know there is caching on the way towards client and it is still sending full data out.
This is working with any TCP based application.
Your proposal would require to convince the server not to send the attachment over WAN
so it can be served from local cache, but that is completely outside of scope of DRE caching.
Thanks