You can use the System.Net.Http.HttpContent.StreamContent instance to add a FileStream object to the to the HttpResponseMessage's Content property. Something like this:
// Return archive stream
HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
result.Content = new StreamContent(fileStream);
result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment") {
FileName = fileName.ToString()
};
Note that you should probably also add a MD5 checksum to the file content in real applications.