Uploading a file using AJAX to ASMX service

Have an input of type="file" which triggers this function:

function DoUpload(files) {
    var reader = new FileReader();
    reader.onload = function (e) {
        var data = {
            file: e.target.result,
            extension: file.name.substr(file.name.lastIndexOf("."))
        };

        $.ajax({
            headers: {
                "X-RequestDigest": $("#__REQUESTDIGEST").val()
            },
            url: "{baseurl}" + "http://ift.tt/1BMmLtZ",
            contentType: "application/json",
            data: JSON.stringify(data),
            dataType: 'json',
            type: "POST",
            success: function (response: ITempDocumentInfo) {
                dfd.resolve();
            },
            error: function (e) {
                console.error(e);
            }
        });
    };
    reader.readAsArrayBuffer(file);
}

The service lives in SharePoint and looks like this:

[WebService(Namespace = "http://www.blah.com/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
[System.Web.Script.Services.ScriptService]
public class FileInfo : System.Web.Services.WebService
{
    [WebMethod(EnableSession = true)]
    public TempDocumentInfo AddScannedItem(string instanceId, string uri, string itemKey, string data, string extension)
    {
        byte[] bytes = new byte[data.Length * sizeof(char)];
        System.Buffer.BlockCopy(data.ToCharArray(), 0, bytes, 0, bytes.Length);
        var stream = new MemoryStream(bytes);
        .
        .
        .
    }
}

Once I have the data in C#, I cannot seem to be able to be able to do anything with it. For example. Uploading and processing a bmp image throws an exception:

var blah = System.Windows
                 .Media
                 .Imaging
                 .BmpBitmapDecoder
                 .Create(stream,
                         BitmapCreateOptions.PreservePixelFormat,            
                         BitmapCacheOption.None);

Am I not generating the AJAX request correctly?