something that ends with an "_as8" or "_aes17" or whatever are references to shaders, and not individual textures.
So you'll have to open up TRE Explorer and look up the shader, it's referencing, and then see what texture(s) the shader uses, and for what purpose.
Knowing what .eft or "effect" file it's using, and what it does, can tell you alot about the shader's function.
"_as8" is typically a shader that is a single flat texture. "_placeholder_as8.sht" is a shader that references only one texture: "_placeholder.dds"
And uses the effect file: "a_simple.eft"
Everything that uses a single flat texture in the game, will typically have a shader like this.
Something that ends with "_hcsb21" Is a more complex shader that allows the object to be colored with a pallet (typically armor), while simultaneously allowing specular ("shiny") effects and a Bumpmap to give it extra detail and depth, this is what "h_color2_specmap_bump.eft" is used for.
Keep in mind that the ".dds" (texture) files need to also make proper use of alpha/color channels accordingly for the desired effect/color pallet to be shown in game.
Hope that helps just a little bit.