What is quickskydark_e.dds?

Located in : \data\textures\cubemaps\

Is this for metallic armor's "shininess"?

Reason I ask Is that I'm working on a new mod video - and a fair amount of times I'm asked to overwrite this one file.

It's one of many cubemaps used for items' reflectivity.

I don't know off the top of my head which items use it specifically.

Why are you being asked to overwrite it? Does a mod (or mods) include the file? Which mod(s)?

yes, the mod included the file - one of the new mods out today caled "Templar set"

Well I went ahead and took a look at the vanilla file and the file in the mod... the author has definitely made no attempt to keep it in line with the vanilla file at a higher quality. It's quite different.

It would have been good form to point the models to a uniquely-named cubemap to ensure compatibility with any other mod that might depend on that file being vanilla.

That said, choice of cubemap isn't a huge deal insofar as how a model looks (it's essentially just the image that gets reflected by a model), so... probably not something that would noticeable in-game without really looking for it?