基本问题似乎是Classic .NET WebAPI无法在其最新版本（v5.2.3）中处理分块的请求。我将假设这是一个错误，并且暂时可以降级到v5.0。

The underlying problem appears to be that the Classic .NET WebAPI fails to process chunked requests in it current newest version (v5.2.3). I'm going to assume that's a bug and as a work-around downgrade to v5.0 for the time being.

特别感谢@Developer提供有用的输入。

Special thanks to @Developer for helpful input.

-S