Close enough for government work (because the {HTTP_REFERER} is notoriously unreliable). Your code says:
If the referrer is not empty AND
If the referrer is not (your domain with optional www) AND
If the file extension is (in the list), FAIL the request.
Question: WHY the NC flag in the RewriteRule? Have you mixed the capitalization of your file extensions? If not, get rid of the No Case flag as it’s not necessary.