Trying to create a texture set for a model.

The model uses custom textures. I can set the normal map, and my modified diffuse map is fine. But the model uses Beth's ShinyDull environment map.

So I unpacked the BSA. I'm using Mod Organizer, so I unpacked it into a new mod folder. Restarted the CK and I can browse to the cubemap folder and pick the file I need. But when I try to select it it tells me

"shinydull_e.dds Path does not exit. Check the path and try again"

So, it's one of those whiskey-tango-foxtrot moments. I mean, of course the path exists. If it didn't I couldn't browse to it!

Any ideas? I can work around it by copying the nif, but it would be nice to at least understand what's happening here.