Tumbler does not generate webm thumbnails.
The WebM is a mkv container, and tumbler can make mkv thumbnails, so I suppose making mkv ones it will be not very difficult (if you rename a webm file to a mkv, then the thumbnail are generated).
Created attachment 5244
enable video/webm in ffmpeg plugin
Here's a proposed patch which enables MIME type video/webm in ffmpeg-thumbnailer plugin. I tested it in Xfce 4.10, Arch Linux and it works OK.
.../ffmpeg-thumbnailer-provider.c | 1 +
1 file changed, 1 insertion(+)
This patch helped in the following case:
Pushed to master:
Author: Jannis Pohlmann <firstname.lastname@example.org>
Date: Fri Jul 25 14:08:03 2014 +0200
Enable support for video/webm in ffmpeg thumbnailer plugin (bug #10360)
Patch provided by email@example.com.