file download via `download` attr. or query-string `?dl`

[xerc]

https://developer.mozilla.org/de/docs/Web/HTML/Element/a#attr-download ; same DOMAIN issue if CDN is used

APACHE expl. for query-parameter dl

  RewriteCond %{QUERY_STRING} ^dl
  RewriteRule ^((.*/)*([^/]+\.[a-z]{2,4}))$ - [E=dlfilename:$3,E=fullpath:$1]
  Header onsuccess set Content-Disposition "attachment;filename=\"%{dlfilename}e\"" env=dlfilename
  Header onsuccess set Content-Location "/%{fullpath}e" env=dlfilename
  Header onsuccess set Link "</%{fullpath}e>; rel=\"canonical\"" env=dlfilename