This seems to be similar to those cases where the person was using a Photoshop plugin of some sort?
The layer names ended up with the file extensions, which makes their way into Spine.
On export, the texture packer strips the extensions, but Spine's skeleton data exporter doesn't. So it causes a mismatch.
mmm...
The immediate fix is for you to open it in Spine and use Find and Replace to find all the ".png" paths and remove them.
(ie, replace ".png" with "")
Then re-export.
(I'll move this thread to Editor in a few days.)