def get_fields(self):
        fields = super(RasterModelSerializer, self).get_fields()
        if not self.Meta.raster_field:
            for name, field in fields.items():
                if isinstance(field, serializers.FileField):
                    self.Meta.raster_field = name
                    break
        fieldname = self.Meta.raster_field
        request = self.context.get('request')
        renderer = getattr(request, 'accepted_renderer', None)
        try:
            obj = self.instance[0]
        except (IndexError, TypeError):
            obj = self.instance
        modelfield = getattr(obj, fieldname, None)
        if (isinstance(renderer, BaseGDALRenderer)
                or not isinstance(modelfield, FieldFile)):
            fields[fieldname] = serializers.ReadOnlyField()
        return fields 

def test_should_file_convert_string():
    assert_conversion(serializers.FileField, graphene.String)