Htaccess - Map dynamic URL query to a static file

I'm trying to map this dynamic query: /wp-admin/admin-ajax.php?action=mymail_form_css to this static file: /wp-admin/mymail_form.css What I've tried so far which doesn't seem to work is this htaccess code in /wp-admin/.htaccess RewriteEngine on RewriteCond %{QUERY_STRING} action=mymail_form_css RewriteRule ^(.*)$ /mymail_form.css [L,R=301] I've also tried placing that in the site's document root and no go. I'm doing this for performance reasons. The dynamic query takes an excessive amount of time to generate a small bit of CSS which hardly ever changes.