function getVideoURI(base)
{
    var extension = '.mp4';

    var videotag = document.createElement("video");

    if ( videotag.canPlayType )
    {
      if (videotag.canPlayType('video/webm; codecs="vp9, opus"') )
      {
          extension = '.webm';
      }
    }

    return base + extension;
}
Returns the URL of a supported video source based on the user agent
@param {string} base - media URL without file extension
@returns {string}