Just to clarify...MediaTypeMapping is not used during the process of reading a request. It is one of the criteria used by the default con-neg algorithm while deciding to write the response. 

The default con-neg algorithm depends on the incoming request's content-type header to decide about the formatter to read. So, in your case you could create a custom formatter which supports the mediatype "application/octect-stream" and add it to the Formatters collection on the config object.

NOTE: i have a crude example formatter here where i check for the body content to start with "[" and end with "]". The following example is written  as a test in XUnit. Hope this helps. Here Test1 fails as its body is not in expected format, where as Test2 passes.

using System;
    using System.IO;
    using System.Net;
    using System.Net.Http;
    using System.Net.Http.Formatting;
    using System.Net.Http.Headers;
    using System.Web.Http;
    using System.Web.Http.ModelBinding;
    using System.Web.Http.SelfHost;
    using Xunit;

    public class ConNegTests : IDisposable
    {
        [Fact]
        public void Test1()
        {
            HttpClient httpClient = new HttpClient();

            HttpRequestMessage request = new HttpRequestMessage();
            request.Content = new StringContent("Hello");
            request.Content.Headers.ContentType = new MediaTypeHeaderValue("application/abc");
            request.RequestUri = new Uri(this.BaseAddress + "/UpdateData");
            request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/abc"));
            request.Method = HttpMethod.Post;

            HttpResponseMessage response = httpClient.SendAsync(request).Result;

            Assert.NotNull(response.Content);
            Assert.NotNull(response.Content.Headers.ContentType);
            Assert.Equal<string>("application/abc", response.Content.Headers.ContentType.MediaType);
            Assert.Equal<string>("[Hello-updated]", response.Content.ReadAsStringAsync().Result);
        }

        [Fact]
        public void Test2()
        {
            HttpClient httpClient = new HttpClient();

            HttpRequestMessage request = new HttpRequestMessage();
            request.Content = new StringContent("[Hello]"); //NOTE
            request.Content.Headers.ContentType = new MediaTypeHeaderValue("application/abc");
            request.RequestUri = new Uri(this.BaseAddress + "/UpdateData");
            request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/abc"));
            request.Method = HttpMethod.Post;

            HttpResponseMessage response = httpClient.SendAsync(request).Result;

            Assert.NotNull(response.Content);
            Assert.NotNull(response.Content.Headers.ContentType);
            Assert.Equal<string>("application/abc", response.Content.Headers.ContentType.MediaType);
            Assert.Equal<string>("[Hello-updated]", response.Content.ReadAsStringAsync().Result);
        }

        private HttpSelfHostServer server = null;

        public ConNegTests()
        {
            HttpSelfHostConfiguration config = new HttpSelfHostConfiguration(this.BaseAddress);
            config.Routes.MapHttpRoute("Default", "{action}", new { controller = "ConNegTests" });

            //NOTE
            config.Formatters.Add(new AbcFormatter());
            config.ServiceResolver.SetService(typeof(IRequestContentReadPolicy), new ReadAsSingleObjectPolicy());

            server = new HttpSelfHostServer(config);
            server.OpenAsync().Wait();
        }

        public void Dispose()
        {
            if (server != null)
            {
                server.CloseAsync().Wait();
            }
        }

        protected virtual string BaseAddress
        {
            get { return string.Format("http://{0}:9090/ConNegTests", Environment.MachineName); }
        }
    }

    public class ConNegTestsController : ApiController
    {
        [HttpPost]
        public string UpdateData(string input)
        {
            return input + "-updated";
        }
    }

    public class AbcFormatter : BufferedMediaTypeFormatter
    {
        public AbcFormatter()
        {
            this.SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/abc"));
        }

        protected override bool CanReadType(Type type)
        {
            if (type == typeof(IKeyValueModel))
            {
                return false;
            }

            return true;
        }

        protected override bool CanWriteType(Type type)
        {
            return true;
        }

        protected override object OnReadFromStream(Type type, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext)
        {
            string content = null;

            using (var reader = new StreamReader(stream))
            {
                content = reader.ReadToEnd();
            }

            if (!content.StartsWith("[") && !content.EndsWith("["))
            {
                throw new HttpResponseException("Invalid data sent. Expected data to begin with '[' and end with ']'");
            }

            string trimmedContent = content.TrimStart('[').TrimEnd(']');

            return trimmedContent;
        }

        protected override void OnWriteToStream(Type type, object value, Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext, TransportContext transportContext)
        {
            var output = "[" + value.ToString() + "]";
            var writer = new StreamWriter(stream);
            writer.Write(output);
            writer.Flush();
        }
    }

    public class ReadAsSingleObjectPolicy : IRequestContentReadPolicy
    {
        public RequestContentReadKind GetRequestContentReadKind(System.Web.Http.Controllers.HttpActionDescriptor actionDescriptor)
        {
            return RequestContentReadKind.AsSingleObject;
        }
    }