Disallow attachment hotlinking (htaccess)

[squishi]

Another forum has hotlinked an attachment image of my forum.
Here are my htaccess rules. They don't work.

#hotlinking rules
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain\.com [NC]
RewriteCond %{HTTP_REFERER} !^http://(www\.)?subdomain\.mydomain\.com [NC]
RewriteRule \.([Gg][Ii][Ff]|[Jj][Pp][Gg]|flv|swf)$ http://mydomain.com/nohotlinking.jpeg [NC,R,L]

#attachments
RewriteCond %{HTTP_REFERER} !^http://(www\.)?mydomain\.com [NC]
RewriteRule attachment\.php\?attachmentid=\d+(\&d\=\d+)?$ http://mydomain.com/nohotlinking.jpeg [NC,R,L]

The attachment image is still loading on the other site.

Could it be that no referer is sent if an attachment image is embedded on a site as an image?
But the second check does not allow an empty referer. So either way, the hotlinking image should be shown instead of the attachment...