code: function checkIsImage (src, onResult) {
    var request;
  
    if (src.tagName) {
      onResult(src.tagName === 'IMG');
      return;
    }
    request = new XMLHttpRequest();
  
    // Try to send HEAD request to check if image first.
    request.open('HEAD', src);
    request.addEventListener('load', function (event) {
      var contentType;
      if (request.status >= 200 && request.status < 300) {
        contentType = request.getResponseHeader('Content-Type');
        if (contentType == null) {
          checkIsImageFallback(src, onResult);
        } else {
          if (contentType.startsWith('image')) {
            onResult(true);
          } else {
            onResult(false);
          }
        }
      } else {
        checkIsImageFallback(src, onResult);
      }
      request.abort();
    });
    request.send();
  }
docs: Call back whether `src` is an image.
  
  @param {string|Element} src - URL or element that will be tested.
  @param {function} onResult - Callback with whether `src` is an image.