You have to install ffmpeg on your server, add the video-thumbnail-context.xml.sample and video-transformation-context.xml.sample files to your extension folder (remove the .sample extension) and put the properties mentionned at the beginning of those files in your alfresco-global.properties file.
I am getting error Failure: Transformer for 'video/x-ms-wmv' source mime type and 'video/x-msvideo' target mime type was not found. Operation can't be performed