function send() {
  var superBuffer = new Blob(recordedBlobs, {type: 'video/webm'});
  $.ajax({
    url: '',
    method: 'post',
    data: {
        'video': superBuffer,
        'csrftoken': csrftoken,
    },
    dataType: 'json',
    success: function(data) {
        console.log(data)
    },
    error: function(error) {
        console.log(error)
    }
  });
}

TypeError: 'slice' called on an object that does not implement interface Blob.

function download() {
  var blob = new Blob(recordedBlobs, {type: 'video/webm'});
  var url = window.URL.createObjectURL(blob);
  var a = document.createElement('a');
  a.style.display = 'none';
  a.href = url;
  a.download = blob;
  document.body.appendChild(a);
  a.click();
  setTimeout(function() {
    document.body.removeChild(a);
    window.URL.revokeObjectURL(url);
  }, 100);
}