Apache反向高速缓存代理 - 为什么不进行高速缓存？

I am attempting to set up a reverse caching proxy for ad graphics (gif, jpg, swf) serving. The proxying is working fine and sending the request on to the origin server, but I can't seem to figure out why the content isn't being cached. My current config is below. The goal is to cache all requests that match the /ca/ URI prefix. The origin server is serving the files with clean URLs, no file extensions, Cache-control max-age=1 week set on the origin server headers.I am attempting to set up a reverse caching pro