In an effort to answer this I created this:
http://www.nearby.org.uk/kml-headers.php
which allows testing of the various options. It contains all the ones
I can think of that might effect it. Let me know if want to see
others, or if want the source...
I cant find a method of doing what you want. The issue seems to be
that many browsers by default show the dialog and have 'always ask for
this type of file' ticked - leaving the option in the users control.
It seems you can force the display of the dialog with Content-
Disposition but not force it to be skipped.
(BTW LiveHTTPHeaders or Fiddler type applications are useful for
testing this... )