Post JSON and file data and from JavaScript to MVC Web API Controller

I want to upload a large file in chunks with JavaScript - i.e. using the File API to slice it, then post the slice blob to the server. I can do this like so:

Javascript:

// file is a file from an file select input element
var data = new FormData();
data.append('Slice', file.slice(start, end));
$http.post('/api/upload', data);  //I'm using AngularJS

MVC Web API Controller:

public async Task Post()
{
    var provider = new MultipartMemoryStreamProvider();
    await Request.Content.ReadAsMultipartAsync(provider);
    var stream = provider.Contents.First().ReadAsStreamAsync().Result;

Without the file slice, I can easily post JSON, and use model binding to pick it up:

Javascript:

var model = {};
model.CurrentBlock = 0;
model.FileName = file.name;
$http.post('/api/upload', model);

Server:

public class UploadModel
{
    public string CurrentBlock { get; set; }
    public string FileName { get; set; }
}

public async Task Post(UploadModel model)
{
}

What I can't figure out how to do both in a single request - post a file slice AND some JSON metadata. I can't seem to create any JSON object of model properties and blob data that will get picked up by any combination of controller method parameters for model binding. I tried things like:

public async Task<UploadModel> Post(UploadModel model, HttpPostedFileBase file)

Or adding the file blob directly to the model:

var model = {};
model.CurrentBlock = 0;
model.FileName = file.name;
model.Payload = file.slice(start, end)
$http.post('/api/upload', model);

But it never comes through. I'm sure I'm missing something simple.