title: dd($request->country_flag); returns null in laravel
body: <p>This is my form to upload a file:</p>
  
  <pre><code>&lt;div class="col-lg-6 col-lg-offset-3"&gt;
  &lt;form method="post" action="{{ route('admin.country.store') }}" enctype="multipart/form-data"&gt;
  &lt;input type="hidden" name="_token" value="{{ csrf_token() }}"&gt;
      &lt;div class="form-group"&gt;
          &lt;label for="country_id"&gt;Country ID&lt;/label&gt;
          &lt;input type="number" class="form-control" id="country_id" name="country_id"&gt;
      &lt;/div&gt;
      &lt;div class="form-group"&gt;
          &lt;label for="country_name"&gt;Country name&lt;/label&gt;
          &lt;input class="form-control" type="text" id="country_name" name="country_name"&gt;
          &lt;p class="danger"&gt;{{ $errors-&gt;first('country_name') }}&lt;/p&gt;
      &lt;/div&gt;
      &lt;div class="form-group"&gt;
          &lt;label for="alternate_title"&gt;Alternate Title&lt;/label&gt;
          &lt;input class="form-control" type="text" id="alternate_title" name="alternate_title"&gt;
          &lt;p class="danger"&gt;{{ $errors-&gt;first('alternate_title') }}&lt;/p&gt;
      &lt;/div&gt;
      &lt;div class="form-group"&gt;
          &lt;label for="country_flag"&gt;Country Flag&lt;/label&gt;
          &lt;input class="" type="file" id="country_flag" name="country_flag"&gt;
          &lt;p class="danger"&gt;{{ $errors-&gt;first('country_flag') }}&lt;/p&gt;
      &lt;/div&gt;
      &lt;div class="btn-group" role="group"&gt;
          &lt;button class="btn btn-default" type="reset"&gt;Reset&lt;/button&gt;
          &lt;button class="btn btn-success" type="submit"&gt;Upload&lt;/button&gt;
      &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p></p>
  
  <p>This is a function in my controller to handle form request.</p>
  
  <pre><code>public function store(Request $request)
  {
      $new_country = new SelectCountry();
      $message = [
          'required' =&gt; "This field can not be empty",
      ];
  
      $this-&gt;validate($request, [
          'country_name' =&gt; 'required',
          'alternate_title' =&gt; 'required',
          'country_flag' =&gt; 'required',
      ], $message);
  
      dd($request-&gt;country_flag);
  }
  </code></pre>
  
  <p>When I do <code>dd($request-&gt;country_flag);</code>, it returns <code>null</code>. It seems like file is not uploaded by the form.</p>
  
  <p>What am I doing wrong?</p>