extjs Combo下拉文本的tooptip

xtype: "combo", fieldLabel: "插入", readOnly: true, mode: "local",
store: (globel.Store = new Ext.data.Store({
proxy: new Ext.data.HttpProxy({ url: "Module/Sms/SmsHandler.ashx" }),
reader: new Ext.data.JsonReader({ root: "data" }, [{ name: "Key" }, { name: "Value" }]),
autoLoad: true
})), valueField: "Value", displayField: "Key", triggerAction: "all", width: 120, matchFieldWidth: true,
tpl: '<tpl for="."><div class="x-combo-list-item" ext:qtitle="" ext:qtip="{Key}">{Key}</div></tpl>',