If you follow this article https://symfony.com/doc/current/form/create_form_type_extension.html you'll see that:

1. In the ImageTypeExtension class, you must define which type are you extending with the getExtendedType() method.
2. When defining a service, you must repeat that information in the extended_type attribute of the form.type_extension tag.

I have two proposals:

1. Why repeat the information in the tag? Please remove this requirement. Thanks!
2. I guess there will be some reasons for this but, why must I define the extended type in the getExtendedType() method instead of just extending the class of that type?