1，图片
var result = new HttpResponseMessage(HttpStatusCode.OK)
{
Content = new ByteArrayContent(stream)
};
result.Content.Headers.ContentType = new MediaTypeHeaderValue("image/jpeg");
2，文件
HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
response.Content = new StreamContent(stream);
response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
{
FileName = "Wep Api Demo File.zip"
};