Ah … Aha … I just watched this … Creating and Using Spritesheets in LibGDX - YouTube
“Texture atlas” (.atlas) is the libgdx term for the image_name_meta.txt that I sketched…
“Sprite sheet” is the thing that in jME is called “Texture atlas”. 
EDIT:
Saw your text popping up at the same time. :chimpanzee_grin:
Okay … so you see how important context and exact words are…
I doubt that jME has support for .atlas files (and does not seem to have a concept for that either).
You could write some custom code that reads .atlas files (should be quite easy).
.fnt files is very similar in concept - it is very similar to .atlas files, but comes from a different corner. I’m currently working on a .fnt2 format that might be “missused” as .atlas files. But still those are all font-related. My .fnt2 files will also contain information that .atlas files contain (names for regions, image format and even sub-channel format).
In the end - it’s just some small text data written in a .atlas (or .fnt) file - and a large texture, that contains many smaller texture regions (in computer sience this is called “texture atlas” for 3d and “sprite sheet” for 2d)…