Since I updated to Yosemite, the .mov previews in Finder no longer show up for videos with an older codec. (When I double click the video, it starts a conversion process.) Is there any way I can see previews of my videos without needing to convert them all? Even if there's a software I can download to see the previews, that is fine. It's too much work to convert all of the videos to see what they are.
Not sure about the Finder preview function. You would have to find a preview handler written for that specific codec (whatever it might be) and compatible with Yosemite. A bit of searching might find that, if it even exists.
If all you want to do is play the video try VLC at videolan.org it's free and well supported and will not try and covert it, just play it as is.