Since this question is for Mavericks, you can do:
sudo mv /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuickLookUI.framework/Versions/A/PlugIns/Movie.qldisplay /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuickLookUI.framework/Versions/A/PlugIns/AVFoundationMovie.qldisplay && ln -s /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuickLookUI.framework/Versions/A/PlugIns/LegacyMovie.qldisplay /System/Library/Frameworks/Quartz.framework/Frameworks/QuickLookUI.framework/PlugIns/Movie.qldisplay
On Mavericks, this will bring back animated QuickLook previews for any format that has a QuickTime plugin.
For some reason, Apple left a QuickLook Display plugin in Mavericks called
LegacyMovie.qldisplay which uses QuickTime instead of AVFoundation, and thus supports more types of media. However, QuickLook doesn't use this plugin, it uses the AVFoundation-based
The above terminal command renames
AVFoundationMovie.qldisplay (so you have a backup) and then symlinks
Movie.qldisplay, so QuickLook will use the QuickTime-based version. (
LegacyMovie.qldisplay will still be available at its original location for anything that actually uses it. I am not clear as to whether anything does.)
With the above command and my updated FFusion QuickTime plugin, I'm able to view H265 and even VP9 videos in QuickLook! (As long as the video is 1080p or below; even unmodified QuickLook doesn't appear to like 4K regardless of the codec.)
I think this might work on slightly newer OSs? I'm not sure when Apple got rid of
LegacyMovie.qldisplay. It definitely won't work on Catalina, since the QuickTime framework is 32bit. Note that on 10.11 and above, you would need to (temporarily) disable System Integrity Protection.
Sorry I saw this question eight years too late!