Sorry Hans, the MTL file is useful to find which part is the mirror, but it's not the MTL file that must be modified.
oxygen, you should export your 3D model first at OBJ + MTL format (both are text files you can edit with any text editor).
Then in the generated MTL file, search for the name of the material following newmtl that match the texture or the color of the mirror in your 3D model.
Once found, search the line where this material name is found in the OBJ file and search back the closest line starting by "o" or "g" in the previous lines. Following "o" or "g", there will be the name of a part of your 3D model that you should prefix with sweethome3d_window_mirror.
For a real example, please have a look to rectangularMirror.mtl and rectangularMirror.obj showing the following mirror:
Finally, note that the reflection effect in the mirror works only at the two best rendering quality levels in Photo creation dialog box.
----------------------------------------
Emmanuel Puybaret, Sweet Home 3D creator
