From http://weblog.west-wind.com/posts/2013/Dec/13/Accepting-Raw-Request-Body-Content-with-ASPNET-Web-API

NakedBodyParameterBinding.cs

/// <summary>
/// Reads the Request body into a string/byte[] and
/// assigns it to the parameter bound.
/// 
/// Should only be used with a single parameter on
/// a Web API method using the [NakedBody] attribute
/// </summary>
public class NakedBodyParameterBinding : HttpParameterBinding
{
    public NakedBodyParameterBinding(HttpParameterDescriptor descriptor)
        : base(descriptor)
    {

    }
    public override Task ExecuteBindingAsync(ModelMetadataProvider metadataProvider,
                                                HttpActionContext actionContext,
                                                CancellationToken cancellationToken)
    {
        var binding = actionContext
            .ActionDescriptor
            .ActionBinding;

        if (binding.ParameterBindings.Length > 1 || 
            actionContext.Request.Method == HttpMethod.Get)            
            return EmptyTask.Start();            

        var type = binding
                    .ParameterBindings[0]
                    .Descriptor.ParameterType;

        if (type == typeof(string))
        {
            return actionContext.Request.Content
                    .ReadAsStringAsync()
                    .ContinueWith((task) =>
                    {
                        var stringResult = task.Result;
                        SetValue(actionContext, stringResult);
                    });
        }
        else if (type == typeof(byte[]))
        {
            return actionContext.Request.Content
                .ReadAsByteArrayAsync()
                .ContinueWith((task) =>
                {
                    byte[] result = task.Result;
                    SetValue(actionContext, result);
                });
        }

        throw new InvalidOperationException("Only string and byte[] are supported for [NakedBody] parameters");
    }

    public override bool WillReadBody
    {
        get
        {
            return true;
        }
    }
}