Replace the Defaults:
RewriteRule \.js$ - [L]
RewriteRule \.css$ - [L]
RewriteRule \.png$ - [L]
RewriteRule \.jpg$ - [L]
RewriteRule \.gif$ - [L]
RewriteRule \.php$ - [L]
with:
RewriteCond %{REQUEST_URI} !(\.gif)|(\.jpg)|(\.png)|(\.css)|(\.js)|(\.php)$
They are functionally equivalent (to my knowledge), the later works on a wider range of Apache versions.