webm video wont run with wordpress

I have manually uploaded a webm video to my server on the wp-content/uploads/2013/08 directory. I can’t upload it to the wordpress library because wordpress tells me that there is a security problem.

Then I insert the video.js short code into my post, and give a url like: “http://ecole-dupuis.ch/wp-content/uploads/2013/08/nameofthevideo.webm“. Unfortunately, when I preview the post, the video player shows, but with no video in it.

Thanks in advance for your help.