Hi,
From your scene I can see a couple things,
1) the expression in the file node is sourceimages/<att:planet default:moon>.jpg instead of sourceimages/<attr:planet default:moon>.jpg
2) the custom attribute added to the sphere in the scene is incorrect, it should be mtoa_constant_planet, you have mtoaconstantplanet, note the underscores in the name of the correct one.
I tested several times,
I finded the reasion. in my case, The resolution of two textures is differenct.
if the resolution of two textures is the same, the token works correctly.