Need help with RSpec xhr request

I have the following function in JS for a drag and drop file uploader:

function uploadFile(file){

             var xhr = new XMLHttpRequest();
             var upload = xhr.upload;
             var uri =
"/imagebooks/upload_supporting_image/?authenticity_token=" +
$('meta[name=csrf-token]').attr("content");

             xhr.open('POST', uri, true);
             xhr.setRequestHeader('X-Filename', file.fileName);

             xhr.send(file);

    }

This works properly. I'm trying to create a spec to test the function
with rspec- anyone know how I can mimic this? I've tried:

before(:each) do
    @image = File.open("#{Rails.root}/public/images/test/test.jpg",
'rb')

  end

  describe 'Upload Supporting Image method' do
    it 'should read a binary string and write a file with it' do
      request.env['X-Filename'] = 'test.jpg'
      xhr :post, :upload_supporting_image, @image
    end
  end

But it doesn't work as expected. Any help is greatly appreciated!