My team has been moving spaces from old server instances into our DC instance, and this "attachments copied by default" business is a disaster.

For example, one space before cleanup was 340 gig (yes, that's the real number!).

We created a ScriptRunner script to delete attachments not referenced on the pages, and that same space after cleanup was 11 gig.

Needless to say, after cleaning up all 96 spaces to be moved, that system was a tiny fraction of it's former size.

Users do NOT understand the current attachment copy process, and it is not fair to expect them to.

Ideally, copyAttachments would be unchecked by default, or administrators would have a configuration option for this.

For completeness as I stumbled over a similar solution here; https://confluence.atlassian.com/confkb/how-to-uncheck-notify-watchers-check-box-on-confluence-editor-by-default-314449908.html 

Just add the follwing JavaScript to the Custom HTML, End of Head area, which unchecks the checkbox after 500ms.

<script>
function disableCopyAttachments() {
  setTimeout(disableCopyAttachments, 500);
  if ( $('#copy-attachments').is(':checked')) {
    setTimeout(function() {$('#copy-attachments').removeAttr('checked');}, 500);
  }
}
disableCopyAttachments();
</script> 

Fully agreed on this one - maybe not even disable, but at least set it to unchecked per default!

Despite the trainings my users got to uncheck this checkmark and only use it when they really need it, they tend to forget it. - And as they copy their old jour-fix page for a new JF they start duplicating attachments of the original page ending up with tens or hundreds of attachments behind their newly created page, hardly using a handful of them....