HI， I get the same problem.

I  find one solution to solve it. 

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Open file view_form.js (../addons/web/static/src/js/view_form.js)  , Then find this code

   on_file_delete: function (event) {
        event.stopPropagation();
        var file_id=$(event.target).data("id");
        if (file_id) {
            var files=[];
            for(var i in this.get('value')){
                if(file_id != this.get('value')[i].id){
                    files.push(this.get('value')[i]);
                }
                else if(!this.get('value')[i].no_unlink) {    #  remove '!'  then delete method will be ok
                    this.ds_file.unlink([file_id]);
                }
            }
            this.set({'value': files});
        }

near line 5348