For use with pre-MVC6 Web API controllers

MultipartFormDataValueProvider.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Net.Http;
using System.Web.Http.Controllers;
using System.Web.Http.ValueProviders;

public class MultipartFormDataValueProvider : IValueProvider
{
    private HttpActionContext actionContext = null;

    private IEnumerable<HttpContent> multipartContents;

    private IEnumerable<HttpContent> MultipartContents
    {
        get
        {
            if (multipartContents == null)
            {
                var task = actionContext.Request.Content.ReadAsMultipartAsync();
                task.Wait();
                multipartContents = task.Result.Contents;
            }

            return multipartContents;
        }
    }

    public MultipartFormDataValueProvider(HttpActionContext actionContext)
    {
        this.actionContext = actionContext;
    }

    public bool ContainsPrefix(string prefix)
    {
        throw new NotImplementedException();
    }

    public ValueProviderResult GetValue(string key)
    {
        var item = MultipartContents.FirstOrDefault(c => c.Headers.ContentDisposition.Name.Trim('"') == key);

        if (item == null)
        {
            return null;
        }

        var task = item.ReadAsStringAsync();
        task.Wait();

        return new ValueProviderResult(
            rawValue: task.Result, 
            attemptedValue: task.Result, 
            culture: CultureInfo.InvariantCulture);
    }
}

public class MultipartFormDataValueProviderFactory : ValueProviderFactory
{
    public override IValueProvider GetValueProvider(HttpActionContext context)
    {
        return new MultipartFormDataValueProvider(context);
    }
}

USAGE.md

You need to register the factory in Startup.cs:

GlobalConfiguration.Configure(config =>
{
    config.Services.Add(typeof(ValueProviderFactory), new MultipartFormDataValueProviderFactory());
}); 

That should take care of complex type arguments on your API controllers. For simple types you have to specify the [ModelBinder] attribute on the parameter.