QuickTime player unfortunately comes with limited codecs/plug-ins so you need to chase them all down. As WW says, VLC is the recommended player on this forum. I use it on my mac. To see about making it your default video player so you can preview files, see How to change default program in OSX

Also, instead of using the Preview option which creates preview files in your incomplete folder, may I recommend instead -> go to LW's Library window, select incomplete folder, arrange by name & find the file you wish to preview & select it, press Launch. This has the advantage of not creating preview files.