Laravel dd($request>all());返回空数组

I am trying to upload a photo from my Laravel 5 app to be stored in AWS. I am using the Postman REST client to test. When I upload a photo, the request returns an empty array. Does anyone know why this might be? Here's the code for my Avatar Controller:

class AvatarController extends Controller
{

  public function __construct(AWS $aws)
  {
      $this->aws = $aws;
  }

/**
 * Store a new avatar for a user.
 * POST northstar.com/users/{id}/avatar
 */
  public function store(User $user, Request $request)
  {
    dd($request->all());
    // dd($request->file('photo'));

    $file = $request->file('photo');
    // $file = Request::file('photo');
    // $file = Input::file('photo');

    $v = Validator::make(
      $request->all(),
      ['photo' => 'required|image|mimes:jpeg,jpg|max:8000']
    );

    if($v->fails())
      return Response::json(['error' => $v->errors()]);         

    $filename = $this->aws->storeImage('avatars', $file);

    // Save filename to User model
    $user->avatar = $filename;
    $user->save();

    // Respond to user with success
    return response()->json('Photo uploaded!', 200);
  }
}