Edit in JSFiddle

Ext.define('Userpanel.controller.Condition', {

    extend: 'Ext.app.Controller',

    init: function() {

        this.control({
            '[itemId="userpanel-widget-condition-field-combobox"]': {
                select: this.onFieldSelect
            },
            '[itemId="userpanel-widget-condition-sign-combobox"]': {
                beforequery: this.beforeSignQuery
            }
        });

    },

    onFieldSelect: function(combobox, records) {

        combobox
            .up('userpanel-condition-widget')
            .down('[itemId="userpanel-widget-condition-sign-combobox"]')
            .getStore()
            .filterByFieldType(records[0].get('type')) // custom filter method
        ;

    },

    beforeSignQuery: function(queryEvent) {

        queryEvent.combo.lastQuery = ''; // http://www.sencha.com/forum/showthread.php?127334-Refresh-ComboBox-after-applying-filterBy-on-Store&p=627369&viewfull=1#post627369

    }

});