title: django get file size and put it in serializer
body: <p><strong>serializers.py</strong></p>
  <pre><code>class ModFileSerializer(serializers.ModelSerializer):
      size = serializers.CharField(default=os.path.getsize('file'), max_length=16)
      class Meta:
          model = models.ModFile
          fields = '__all__'
  
          read_only_fields = (
          'size',
          )
  </code></pre>
  <p><strong>models.py</strong></p>
  <pre><code>class ModFile(models.Model):
      downloads = models.IntegerField(default=0)
      file = models.FileField(upload_to='mods/')
      created_at = models.DateTimeField(auto_now_add=True)
  </code></pre>
  <p>Here I have a serializer for the ModFile model and the only thing that's missing is the file's size, I know os.path.getsize() needs the exact location of the file so how do I actually access the file field from the model in order to pass in the getsize() function or is there a better way to do it?</p>