Uploading Image null on Laravel 5.0

I had created a form like the following in my view

{!! Form::open(array('url' => 'user/posts', 'files'=> true)) !!}
           <div class="form-group">
               {!! Form::text('title',Input::old('title'),['class'=>'form-control', 'placeholder' => 'What is your title?']) !!}
           </div>
           <div class="form-group">
              {!! Form::label('image', 'Your Image') !!}
              {!! Form::file('image') !!}
             <p class="help-block">Hey! Please don't upload over 15MB images!</p>
           </div>
           <div class="form-group">
            {!! Form::label('caption', 'Don\'t miss to caption!') !!}
            {!! Form::text('caption',Input::old('caption'),['class'=>'form-control', 'placeholder' => 'Your caption']) !!}
           </div>
           {!! Form::submit('Post  now!',['class'=>'btn btn-info']) !!}
      {!!Form::close() !!}

And I try dd() for my image from my controller but it return nulll.

public function store(PostFormRequest $request)
    {   
  dd($request->input('image'));
  $post = new Post;
    }

The following one is my PostFormRequest

<?php namespace App\Http\Requests;

use Response;
use Illuminate\Foundation\Http\FormRequest;

class PostFormRequest extends FormRequest {

    public function rules()
    {
        return [
            'title' => 'required',
            'image' => 'mimes:jpeg,bmp,png'
        ];
    }

    public function  authorize()
    {   
     $image = $this->image;
     if(empty($image))
      return true;
     else 
      return false;
    }

    public function forbiddenResponse()
    {
        return Response::make('Sorry!',403);
    }
}