Luca,
Please check the RemoveButtonBehavior property.
by default the value is HideAndUncheck ,
When an user click the remove button ,
the item.Checked would set to false.
So that it would not delete an existing item.
(Please check the advanced sample - forum edit topic)
You can set to AttachmentItemBehavior.Delete for your case.
--
By the way , we will add new value 'Auto' for it.
(HideAndUncheck for custom items, Delete for uploaded items)
Regards,
Terry