title: Slim 3 method getUploadedFiles() returns an empty array
body: <p>When trying to mock a test with a test file I get stuck. Basically the <code>withUploadedFiles()</code> method wants me to work with an array.
  As you can see down below, I try to <code>var_dump</code> to see if it has worked but in every case the array is empty. I wonder why this is?
  If I only <code>var_dump</code> my array with the name <code>$uploadedFiles</code> it shows me the content. </p>
  
  <p>Do you see where my mistake is?</p>
  
  <pre><code>&lt;?php
  
  use Slim\Http\UploadedFile;
  
  class FileControllerTest extends PHPUnit_Framework_TestCase
  {
      // ...
  
      public function testUserCanCreateAFileSuccessfullyWithStatusCode200()
      {
          // ...
  
          $uploadedFile = new UploadedFile(
              $directory . '/File_1.txt',
              'File_1.txt',
              'text/plain',
              filesize($directory . '/File_1.txt'),
              0
          );
  
          $uploadedFiles = array();
          $uploadedFiles["file"] = $uploadedFile;
  
          // ...
  
          $request = \Slim\Http\Request::createFromEnvironment($environment);
          $request-&gt;withUploadedFiles($uploadedFiles);    
  
          var_dump($request-&gt;getUploadedFiles());         // array(0) { }
  
          // ...
      }
  }
  </code></pre>