How to create mipmap for textures loaded with www?

When I load a texture using the code below:

WWW www = new WWW(textureName);     

yield return www;

renderer.material = new Material(Shader.Find("Diffuse"));

Texture tex = www.texture;
tex.filterMode = FilterMode.Trilinear;

renderer.material.mainTexture =tex;

The texture does not come with mipmaps, how can I tell unity to generate the mipmaps?

http://answers.unity3d.com/questions/2814/how-do-i-generate-mipmaps-at-runtime