The most common cause of this problem are MIME type (i.e. Content-types) misconfigurations in your web server. While most web servers are already configured to serve .mp4 files with the proper MIME type, they are often not configured to properly serve .webm.
- .mp4 files should be served with the MIME type video/mp4
- .webm files should be served with the MIME type video/webm