The file isn't being uploaded at all. It should be creating a <filename>_thumb.jpg that is resized off the original full image to a 50x50 size thumb. Nothing is uploaded. This was working when I had both uploads working under the single behavior (EXCEPT) the full image was getting compressed, not untouched. Example.... 300k file was getting compressed to 40k, and a second 50x50 thumbnail (2k) was getting created off the single upload.
See attached file.