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var result = new HttpResponseMessage(HttpStatusCode.OK)
{
   Content = new ByteArrayContent(byteArray)
};

result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
{
    FileName = attachment.FileName
};
result.Content.Headers.ContentDisposition.FileName = attachment.FileName;

return Task.FromResult(result);