For v12 I'm assuming you're following these instructions. Paying close attention to the
PRODUCT_IMAGES_FIELD_GUID field. The documentation states
// Navigate to the 'Page types' application -> Select the Page type -> 'Fields' tab -> Select the field, you get the GUID from that field.
This is assuming you have a field on the page type to accept those attachments and it is using the Attachments Control as the form control. If you're not doing this then you'll get 404 errors because the system can't find the reference to the page and field.
On the front end, since you're using the Grouped Attachment, you'll need to loop through the object and output the items. Here's a reference to a post I did on this a few years ago, but using Portal Engine vs. MVC. The concept is the same, but the language will be different. Here's some documentation on how to handle displaying this in MVC.