Hype uses a service built into Mac OS for generating the video previews, and at this time the webm format is not supported. If your video has both webm and MP4 variants in the same video group, Hype will be able to show the video preview. Chrome will still do the right thing and load the webm video, and you’ll get the benefit of video previews. (You can even use very low resolution mp4 videos for this)
To quickly convert several webm videos, I recommend Miro Video Encoder: http://www.mirovideoconverter.com
Hope that helps!