FileResult

public class FileResult : IHttpActionResult
    {
        private readonly byte[] _fileBinary;
        private readonly string _contentType;
        private string _fileName;

        public FileResult(byte[] fileBinary, string contentType, string fileName)
        {
            this._fileBinary = fileBinary;
            this._contentType = contentType;
            this._fileName = fileName;
        }

        public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
        {
            var response = new HttpResponseMessage(HttpStatusCode.OK);

            //var response = new HttpResponseMessage(HttpStatusCode.OK)
            //{
            //    Content = new StreamContent(File.OpenRead(@"C:\www\templates\test.zip"))
            //};

            response.Content = new ByteArrayContent(this._fileBinary);
            response.Content.Headers.ContentType = new MediaTypeHeaderValue(this._contentType);
            response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
            response.Content.Headers.ContentDisposition.FileName = this._fileName;

            return Task.FromResult(response);
        }
    }