Only inject file_url if it is defined in the manifest config

Author: nadove-ucsc

src/azul/service/manifest_service.py

@@ -1684,7 +1684,8 @@ def write_page_to(self,
                             for related_file in file['related_files']:
                                 related_row = {}
                                 file.update(related_file)
-                                file['file_url'] = self._azul_file_url(file)
+                                if 'file_url' in column_mapping:
+                                    file['file_url'] = self._azul_file_url(file)
                                 self._extract_fields(field_path=field_path,
                                                      entities=[file],
                                                      column_mapping=column_mapping,