title: Django storage S3 - Save File Path only without file using ModelSerializer
body: <p>I'm using boto3 to upload files to S3 and save their path in the FileField. </p>
  
  <pre><code>class SomeFile(models.Model):
      file = models.FileField(upload_to='some_folder', max_length=400, blank=True, null=True)
  </code></pre>
  
  <p>For the above model the following code works to create a record.</p>
  
  <pre><code>ff = SomeFile(file='file path in S3')
  ff.full_clean()
  ff.save()
  </code></pre>
  
  <p>Now, when I use ModelSerializer to do the same.  </p>
  
  <pre><code>class SomeFileSerializer(serializers.ModelSerializer):
      class Meta:
          model = SomeFile
          fields = ('file')
  </code></pre>
  
  <p>I get this error after running the code below </p>
  
  <blockquote>
    <p>rest_framework.exceptions.ValidationError: {'file': [ErrorDetail(string='The submitted data was not a file. Check the encoding type on the form.', code='invalid')]}</p>
  </blockquote>
  
  <pre><code>serializer = SomeFileSerializer(data={'file': 'file path to S3'})
  serializer.is_valid(raise_exception=True)
  </code></pre>
  
  <p>I need help in setting up the serializer to accept file path without actually having the file. </p>