Problems viewing Collada files in Gazebo-ROS

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I have followed the tutorial to insert a Collada .dae file into Gazebo, so that my world file is:

<?xml version="1.0" ?>
<sdf version="1.4">
<world name="test_world">
    <include>
        <uri>model://sun</uri>
    </include>
    <include>
        <uri>model://ground_plane</uri>
    </include>

<model name="duck">
    <pose>0 0 0  0 0 0</pose>
    <static>true</static>
    <link name="body">
        <visual name="visual">
            <geometry>
                <mesh><uri>file://duck.dae</uri></mesh>
            </geometry>
        </visual>
    </link>
</model>
</world>
</sdf>

If I run gazebo test_world.world I can see the duck. However, if I create the following launch file:

<?xml version="1.0" encoding="UTF-8"?>
<launch>

<include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name"   value="$(find my_package)/models/test_world.world" />
    <arg name="use_sim_time" value="true" />
    <arg name="debug"        value="false" />
    <arg name="gui"          value="true" />
</include>
</launch>

The duck is not displayed in Gazebo, although I can see the element exists in the left-hand pannel. I have seen other people with my problem, and tried many things, but I cannot make it work.

Should the .dae and .world files be in a specific folder? Any help is appreciated! (I'm using Gazebo 4.1.3 with ROS Indigo).

UPDATE: After applying the accepted answer, this is the final state (note that file:// has changed to model://)

Launch file:

<?xml version="1.0" encoding="UTF-8"?>
<launch>
<include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name"   value="$(find my_package)/models/test_world.world" />
    <arg name="use_sim_time" value="true" />
    <arg name="debug"        value="false" />
    <arg name="gui"          value="true" />
</include>
</launch>

World file:

<?xml version="1.0" ?>
<sdf version="1.4">
<world name="test_world">
    <include>
        <uri>model://sun</uri>
    </include>
    <include>
        <uri>model://ground_plane</uri>
    </include>

<model name="duck">
    <pose>0 0 0  0 0 0</pose>
    <static>true</static>
    <link name="body">
        <visual name="visual">
            <geometry>
                <mesh><uri>model://duck.dae</uri></mesh>
            </geometry>
        </visual>
    </link>
</model>
</world>
</sdf>

And the folder structure is:

• my_package
  • models
    • test_world.world
    • duck.dae
  • lauch
    • test_launch.launch
  • CMakeLists.txt
  • package.xml