The V2 REST API endpoint GET /wiki/api/v2/custom-content requires type as a mandatory parameter, but there is no companion endpoint to retrieve the list of available custom content types on an instance. 

As a workaround we can omit the type parameter, but it contradicts with the official documentation https://developer.atlassian.com/cloud/confluence/rest/v2/api-group-custom-content/#api-custom-content-get

Request: Introduce one of the following:

1. A new endpoint (e.g., GET /wiki/api/v2/custom-content/types) that returns all registered custom content types on the instance, or
2. Officially support and document the type parameter as optional on GET /wiki/api/v2/custom-content, so that omitting it returns all custom content across all types.