Downloading kml from gwc demo page (http://<host>/geoserver/gwc/demo) returns "geoserver-dispatch.application" file name instead of *.kml.
I got Content-Disposition: inline; filename=geoserver-dispatch.application from response headers as you can see in the attachments.
It's probably similar bug with the issue I cloned ()
Chrome Version 67.0.3396.99 (Official Build) (64-bit)
Opera Version 54.0.2952.51
Windows 10 - 64 bit