Can you try this hasty workaround? procedure TMainForm.UniFormReady(Sender: TObject); begin UniFileUploadButton1.JSInterface.JSCode(#1+ '.fileInputEl.addBeforeListener("change", function(a, b) {'+ ' isValid = true;'+ ' for (i = 0; i < b.files.length; i++) {'+ ' if (b.files[i].size > '#1'.maxSize) {'+ ' isValid = false;'+ ' Ext.MessageBox.show({'+ ' title: "MaxSize = " + '#1'.maxSize,'+