BinaryProcessor.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Nancy;
using Nancy.Responses;
using Nancy.Responses.Negotiation;

namespace ByteArrayDemo
{
    public class BinaryProcessor : IResponseProcessor
    {
        public static IList<Tuple<string, MediaRange>> Mappings { get; set; }

        static BinaryProcessor()
        {
            Mappings = new List<Tuple<string, MediaRange>>();    
        }

        public IEnumerable<Tuple<string, MediaRange>> ExtensionMappings
        {
            get { return Mappings.ToArray(); }
        }

        public ProcessorMatch CanProcess(MediaRange requestedMediaRange, dynamic model, NancyContext context)
        {
            var acceptableType = (model != null && (model.GetType() == typeof(byte[]) || model is Stream));

            var modelResult = acceptableType ? MatchResult.ExactMatch : MatchResult.NoMatch;
            var contentTypeResult = Mappings.Any(map => map.Item2.Matches(requestedMediaRange)) ? MatchResult.ExactMatch : MatchResult.NoMatch;

            return new ProcessorMatch { ModelResult = modelResult, RequestedContentTypeResult = contentTypeResult };
        }

        public Response Process(MediaRange requestedMediaRange, dynamic model, NancyContext context)
        {
            if (model is Stream)
            {
                return new StreamResponse(() => model, requestedMediaRange);
            }

            return new ByteArrayResponse((byte[])model, requestedMediaRange);
        }
    }
}

Bootstrapper.cs

using System;
using Nancy;
using Nancy.Responses.Negotiation;

namespace ByteArrayDemo
{
    public class Bootstrapper : DefaultNancyBootstrapper
    {
        protected override void ApplicationStartup(Nancy.TinyIoc.TinyIoCContainer container, Nancy.Bootstrapper.IPipelines pipelines)
        {
            base.ApplicationStartup(container, pipelines);

            ByteArrayProcessor.Mappings.Add(new Tuple<string, MediaRange>("jpg", "image/jpeg"));
            ByteArrayProcessor.Mappings.Add(new Tuple<string, MediaRange>("jpeg", "image/jpeg"));
            ByteArrayProcessor.Mappings.Add(new Tuple<string, MediaRange>("png", "image/png"));
        }
    }
}

ByteArrayResponse.cs

using System.IO;
using Nancy;

namespace ByteArrayDemo
{
    public class ByteArrayResponse : Response
    {
        /// <summary>
        /// Byte array response
        /// </summary>
        /// <param name="body">Byte array to be the body of the response</param>
        /// <param name="contentType">Content type to use</param>
        public ByteArrayResponse(byte[] body, string contentType = null)
        {
            this.ContentType = contentType ?? "application/octet-stream";

            this.Contents = stream =>
                {
                    using (var writer = new BinaryWriter(stream))
                    {
                        writer.Write(body);
                    }
                };
        }
    }
}

MainModule.cs

using System.IO;
using Nancy;

namespace ByteArrayDemo
{
    public class MainModule : NancyModule
    {
        public MainModule()
        {
            Get["/"] = _ => Negotiate.WithView("index")
                                     .WithMediaRangeModel("image/jpeg", () => File.ReadAllBytes(@"Koala.jpg"))
                                     .WithMediaRangeModel("image/png", () => File.OpenRead(@"rowlf.png"));
        }
    }
}