AjaxButton - image is not shown even though type="image" is in html-template

It seems that in html-template type="image" is changed to type="button".

There is not much to do.
in html:
<form wicket:id="form">
<input wicket:id="button" type="image" src="add.png" value='this must be an image></input>
</form>

in java:
Form form = new Form<>("form");
add(form);
form.add(new AjaxButton("button") {});

As image use any.

In Wicket 6.20.0 it worked.