Revert "fix(server): use thumbnail content type instead of application/octet-stream (#3075)" (#3134)

* Revert "fix(server): use thumbnail content type instead of application/octet-stream (#3075)"

This reverts commit 3cc77d945b.

* generate api

mobile/openapi/test/person_api_test.dart

@@ -32,7 +32,7 @@ void main() {
       // TODO
     });
 
-    //Future getPersonThumbnail(String id) async
+    //Future<MultipartFile> getPersonThumbnail(String id) async
     test('test getPersonThumbnail', () async {
       // TODO
     });