Why the Unity3D only can load 32-bit native plugin(.so) on android platform?

I built a native plugin (shared library, .so) for ARM64-v8 on android platform.

It works well with the plain android apps.

But, when the Unity3D loads it, I just got an error message with "... .so is 64-bit instead of 32-bit....".

I just tried to use il2cpp to build the 64-bit app with Unity3D, but it also doesn't work.

Does the Unity3D not support 64-bit native plugin on android platform?

Or

How can I use it, 64-bit native plugin on android platform?

Please let me get some idea.

Thanks.

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Currently the tools what I use are as follow:

OS - Windows 10 64-bit

Unity3D (Editor?) - 5.5.03f Personal (64-bit)

AndroidNDK64 - android NDK r10e for il2cpp