Get field added via attachment_fields_to_edit filter in Gutenberg

Yes, you’re right that the slimImageObject function is the reason why you’re seeing/getting only those limited list of props.

But don’t worry, you can use wp.data.select( 'core' ).getMedia( <attachment id> ) to get the attribution meta (and any other meta or props in the full attachment object). Here’s an example that worked for me:

items.map((item, index) => {
    // Add this line and the attribution part in the attachment div.
    const attribution = wp.data.select( 'core' ).getMedia( item.id )?.meta?.attribution;

    return (
        <div className="attachment" key={index}>
            <figure>
                <img src={item.url} alt={item.alt} key={item.id} />
            </figure>
            Attribution: { attribution || 'NA' }
        </div>
    )
})