TinyMCE v6 - Insert image as base64

With v5 of TinyMCE I can use the following code to insert an image as a base64 string into the editor:

tinymce.init({
    paste_data_images: true,
    plugins: 'image imagetools',
    selector: '#editor',
    smart_paste: true,
    toolbar: 'image',
    file_picker_types: 'image',
    images_upload_handler: function (blobInfo, success) {
        success('data:' + blobInfo.blob().type + ';base64,' + blobInfo.base64());
    }                  
});

The documentation for v6 specifies images_upload_handler now returns a promise, but I can't get it working. Can anyone point me in the right direction?