The form-data is not a documented feature. It might have worked in the Request Method before platform v2.3. With platform v2.3, the Request Method has been enhanced to support only via defined request templates. Form-data is not one of the supported options.
If we have to support form-data, we have to accept it as a new feature request and validate and support it if feasible and prioritized.
Multipart content type is not supported, and attachments cannot be sent via Request Template. So, please find alternative way to work with attachments.
Refer to this post for the supported content types in the Request Method.