i've been modified code in SkeletonDataCompatibility.cs(line no:94) to fix it:Extension is '.skel.bytes' but content looks like a '.json' file
bool isJsonFileByContent = true;
if ( hasBinaryExtension )
isJsonFileByContent = false;//IsJsonFile(asset);
please recheck it if ( hasBinaryExtension )
isJsonFileByContent = false;//IsJsonFile(asset);
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it's in spine-unity 4.0 Unity package