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). Related: https://bugzilla.xfce.org/show_bug.cgi?id=6486#add_comment
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: https://forum.manjaro.org/index.php?topic=10101.0
Pushed to master: commit ec33c5395438fb4662b17d07d5463483fbdecdc3 Author: Jannis Pohlmann <jannis@xfce.org> Date: Fri Jul 25 14:08:03 2014 +0200 Enable support for video/webm in ffmpeg thumbnailer plugin (bug #10360) Patch provided by kwiatek@iv.pl.