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I'm running Unity 2020.2.0a18 but have had the same problem with earlier versions. I think the problem is with my Player Settings for WebGL. According to the Unity page that describes the WebGL Player settings: (https://docs.unity3d.com/Manual/webgl-building.html) there should be 6 files generated in the Build folder:
But the 4 (only) Build folder files I get are:
So I haven't got enough files. And the ones I do have are the wrong kind :o)
The funny thing is that the resulting web page loads, looks good and seems to be OK (no scripts are running to check behavior) but I'm unable to successfully upload and post it on other WebGL hosting sites.