title: Django Reset Serializer loses a file and raises validation error
body: <p>I have a Simple serializer.</p>
  
  <pre><code>class MySerializer(serializers.Serializer):
      file = serializers.FileField()
  </code></pre>
  
  <p>The main problem is that it raises validation error, even though <strong>I provided the file to serializer</strong>. The view will return message that data are invalid. This behaviour seems to be wrong. The file which I clearly pass to the serialzier magically disapears which results in validation error.</p>
  
  <p>class MyView(APIView):</p>
  
  <pre><code>    permission_classes = ()
      parser_classes = (MultiPartParser,)
  
      def post(self, request):
          print(request.data)
          # &lt;QueryDict: {'file': [&lt;InMemoryUploadedFile: some_file.png (image/png)&gt;]}&gt;
          serializer = MySerializer(data=request.data)
  
          serializer.is_valid(raise_exception=True)
  
          ...
  </code></pre>
  
  <p>The endpoint responses with: </p>
  
  <pre><code>{
    "file": [
      "This field can not be null."
    ]
  }
  </code></pre>
  
  <p>This behaviour is strange to my. The docs says that <code>request.data</code> contains both <code>file</code> and <code>non-file</code> object. We can even see that in my <code>print</code>. The question is why my serialzier can't make use of it? Thie file magically disappears.</p>
  
  <p>I use this serializser in my view:</p>
  
  <pre><code>class MyView(APIView):
  
      permission_classes = ()
      parser_classes = (MultiPartParser,)
  
      def post(self, request):
          print(request.data)
          # &lt;QueryDict: {'file': [&lt;InMemoryUploadedFile: some_file.png (image/png)&gt;]}&gt;
          serializer = MySerializer(data=request.data)
          serializer.is_valid()
  
          print(serializer.data)
          # {'file': None}
  </code></pre>
  
  <p>With a little bit of testing I see that even though I pass data to serializer, which containts a file. But after checking <code>serializer.data</code> it says that <code>file</code> is <code>None</code>.</p>