I have question related to the attachments.
In my case I have a website for used cars and when I built it, years ago, I used a plugin to let my client post from the front-end.
This plugin is no longer working with the latest version of WordPress, I removed this old plugin and I installed the Meta Box plugin.
This old plugin created some custom fields for brand, model, color and other stuff. With meta box I was able to retrieve that information. But I'm wasn't able to get the images for each post.
I was looking at this solution you provided here and using an image field but is not working.
Here you can see a screenshot of the Meta Box working with the custom fields, another meta box with empty images, and the attachments of the post.
What I'm trying to achieve is to put the images in any of the image uploaders available so that my client can later add more images or removed them.
Thanks and let me know if you have any questions for me.