$funcNum = $_GET['CKEditorFuncNum'] ;
is not returning the number.
I tried creating the upload.php for ckeditor uploadimage plugin according to https://stackoverflow.com/a/44553006/8719001 (sorry can't add comments yet)
which includes echo "<script type='text/javascript'>window.parent.CKEDITOR.tools.callFunction($funcNum, '$url', '$message');</script>";
However when I drop images it doesn't work and in the console I get the Response Text which doesn't show any funcNumber:
"<script type='text/javascript'>window.parent.CKEDITOR.tools.callFunction(, 'https://example.com/upload/test.jpg', '');</script>"
I think this might be part of filebrowser plugin?
Which I have enabled and also declared $settings['filebrowserUploadUrl'] =