fixed: Only force cache for video else it may cause a thread-storm

A side effect of the new buffer-all mode (aka always-buffer) is that *every* local is read using our cache. This is causing a thread-storm(tm) like http://forum.xbmc.org/showthread.php?tid=169674&pid=1600317#pid1600317 which obviously was not intended by design and possibly even harms overall performance on eg. ARM systems. IMO always-buffer/cache should only be enabled for files send from DVDPlayer which this PR does.