Edit /etc/httpd/conf/extra/httpd-deflate.conf

Change
.(?:gif|jpe?g|png|rar|zip|exe|flv|swf|mov|wma|mp3|avi|mp?g)$ no-gzip dont-vary
to
.(?:gif|jpe?g|png|rar|zip|exe|flv|mov|wma|mp3|mp4|avi|mp?g)$ no-gzip dont-vary