ProseMirror: How to add an attribute to pasted content?

I want to add an attribute which will be rendered as an HTML attribute to content that was pasted.

I have been trying to implement this using something like the following function:

transformPasted: (slice) => {
    let fragment = slice.content
    fragment.forEach((node, _, index) => {
        fragment = fragment.replaceChild(
            index,
            schema.node(
                node.type,
                {
                    ...node.attrs,
                    pasted: true,
                },
                node.content,
                node.marks,
            ),
        )
    })
    return new Slice(fragment, slice.openStart, slice.openEnd)
},

This does seem to be setting the attribute correctly however it still always displays pasted="false" in the HTML. I haven’t managed to successfully change the HTML attribute to pasted="true".

Any guidance would be appreciated.