Files + Form Data values to WebAPI controller via jQuery using memory only?

Question

The Components

• A form with some values on an asp.net web form
• A file input (standard HTML5, not asp:File or anything like that)
• The bluimp jQuery file uploader which I understand is pretty great for this sort of thing but have never used
• An MVC4 WebAPI controller
• A knockoutJS viewmodel in the web form's JavaScript (shouldn't matter here but worth mentioning).

The Goal

Basically, I have tiny Excel sheets that I'm uploading to validate, process, turn into business objects, and return as JSON items.

I'd like the following to happen:

• User types in some data to the form, etc.
• User adds a file to the form.
• The file is automatically uploaded (using bluimp control) to the MVC controller along with a form field value that contains text.

Next, on the controller,

• the text field passed in is sanitized/validated.
• the file is processed into memory
  • (these are truly tiny and there aren't many of them so I'm thinking we shouldn't need to go to the file system?)
• The file is then processed using a FileProcessor that uses the stream and is instantiated using the text field.

The Problem

I don't understand these components well enough yet to understand if I'm making a simple mistake aor a complex one.

What I Have so Far

NOTE: Was trying this with a button to explicitly click to send the file, though in the future I'd like to do it automatically.

The form to submit the file:

<form id="fileUploadForm" action="/api/InvoiceDetailsFile/PostForProcessing" method="POST" enctype="multipart/form-data">
    <input type ="hidden" name="clientSiteId" id="clientSiteId" value="3"/>
        <input type="file" id="inpFile" required="required"/>
<input type ="button" id="btnSendFile" title="Send File" value="Send File"/>

</form>

The JavaScript for the Upload (this is likely very wrong):

        $(document).ready(function () {

            'use strict';

            var btnSendFile = $("#btnSendFile");
            var frmFileUpload = $("#fileUploadForm");

            btnSendFile.click(function () {
                console.log('send button clicked');
                frmFileUpload.fileupload("send");
            });
 });

The WebAPI Controller to handle the upload (this is likely very wrong):

    [HttpPost]
    public Task<List<IInvoiceDetailItem>> PostForProcessing(int clientSiteId)
    {
        if (!Request.Content.IsMimeMultipartContent())
        {
            throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
        }

        var results = new List<IInvoiceDetailItem>();

        var streamProvider = new MultipartMemoryStreamProvider();
        var task = Request.Content.ReadAsMultipartAsync(streamProvider).ContinueWith(t =>
                {
                    // can I only expect 1 item here?
                    foreach (var item in streamProvider.Contents)
                    {
                        // if it's an Excel Sheet
                        if (item.IsMimeMultipartContent("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"))
                        {
                            try {
                                var workbook = new XLWorkbook(item.ReadAsStreamAsync().Result);

                                // clientSiteId here comes from form value (via constructor?), ContextUser comes from a BaseApi.
                                var processor =
                                    new InvoiceProcessorFactory(clientSiteId, ContextUser) 
                                        .GetInvoiceProcessorInstance();
                                 results = processor.ProcessInvoice(workbook).ToList();
                            }
                            catch (Exception ex)
                            {
                                throw new HttpException(Convert.ToInt32(HttpStatusCode.InternalServerError), "Well, this is a bummer. The invoice processor failed. Please see the inner exception.", ex);
                            }
                        }
                    }

                    return results;
                });
        return task;
    }

I'll continue to update the code samples as I progress but I wanted to get it out there sooner than later since I know I'll be struggling with it.