直接上代码：

let url = ApiUrl +"/Query/download?filePath=" + data.url + "&fileName=" + data.title;
//执行下载
download(url);

function download(getur) {
    //通过http.status状态码返回资源存在情况
    let http = new XMLHttpRequest();
    http.open('GET', getur, false);
    http.send();
    console.log('http.status:',http.status);

    let str = document.createElement("a");//创建a标签
    str.href = getur;
    document.body.appendChild(str);
    str.style.display = "none";//隐藏标签
    str.click();

}

C#服务端代码

private HttpResponseMessage downloadCom(string filePath, string fileName)
        {
            //先要判断下载的文件是否还存在
            if (File.Exists(filePath))
            {
                var stream = new FileStream(filePath, FileMode.Open);
                HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);//这里返回200
                response.Content = new StreamContent(stream);
                response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
                response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
                {
                    FileName = fileName
                };
                return response;
            }
            else
            {
                HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.NotFound);//这里返回404不存在
                return response;
            }
        }