使用 IActionResult 而不是 HttpResponseMessage.并返回 FileStreamResult，并使其正常工作.

used IActionResult instead of HttpResponseMessage. And returned FileStreamResult, and got it working.

遇到一个新问题，该文件不是我用来自服务器的流打开的文件.但是会为此创建一个新问题.

Got a new problem, the file is not the one I open with the stream from server. But will create a new question for that.

继续:从 ASP.NET 5 Web API 返回文件

谢谢