title: DRF - change file field by url
body: <p>I am new to Django Rest Framework. I have a Django model with a file field. </p>
  
  <pre><code>class MyClass(models.Model):
      name = models.CharField(max_length=200, null=True, blank=True)
      file = models.FileField(blank=False, null=True, use_url=True)
  
  class MyClassSerializer(serializers.ModelSerializer):
      class Meta(object):
          model = MyClass
          exclude = []
  
  class MyClassView(generics.RetrieveUpdateDestroyAPIView):
      queryset = MyClass.objects.all()
      serializer_class = MyClassSerializer
  </code></pre>
  
  <p>Take two <code>MyClass</code> instances A &amp; B. From my front-end, I'd like to send a PUT request to copy the file of Instance A to model B. </p>
  
  <p>Therefore I first make a GET requests which returns instance A with a file URL (ex: `{..., file: '<a href="http://myserver.com/file.pdf" rel="nofollow noreferrer">http://myserver.com/file.pdf</a>')). I then send a PUT request to update instance B with that URL but I get the following error:</p>
  
  <pre><code>The submitted data was not a file. Check the encoding type on the form. 
  </code></pre>
  
  <p>For performance reasons I don't want the file to be downloaded and uploaded again. Any idea how to serialize the filed field to make it accept an url of another file on my server ? </p>