Custom Attachments Issues with ActionText/Rails - Youtube Video Embeds

Im using trix in rails 7+ for context.

This has been talked about before, but i'm still struggling. I'm trying to attach a YT video into my Trix Editor/ActionText rich content.

```
let embed = ''
let attachment = new trixInst.Attachment({ content: embed, contentType: 'text/html' })
trixEdtior.editor.insertAttachment(attachment);
```

This doesn't work for some reason. It attaches the video into the editor. But after saving it doesn't seem to convert/save to an ActionTextAttachment?

I thought based on this commit (https://github.com/rails/rails/commit/4499a3cdd0c), we can just set the html as the content as long as we set the `contentType` something custom with HTML?

Do we need to create a partial that matches the contentType? Im not sure. I was able to get something working with SGID's, but I'd really prefer to not have to persist every youtube video into a table record if possible.

I have also whitelisted the Iframe Tag.

Do we still need to do this with SGIDS? and `ActionText::Attachable`?

Start with the Trix Attachment API entry points shown in the example and compare their ActionText/Rails serialization path with commit 4499a3cdd0c. Check whether a custom contentType is persisted and rendered without an SGID or matching partial; done means the supported path for YouTube iframe embeds is documented or the limitation is clearly established.

Written by the indexing model from the issue text.