The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
const obj = this._catDict.get("Names");
    let attachments = null;
    if (obj instanceof Dict && obj.has("EmbeddedFiles")) {
      const nameTree = new NameTree(obj.getRaw("EmbeddedFiles"), this.xref);
      for (const [key, value] of nameTree.getAll()) {
        const fs = new FileSpec(value, this.xref);
        if (!attachments) {
          attachments = Object.create(null);
        }
        attachments[stringToPDFString(key)] = fs.serializable;
      }
    }
    return shadow(this, "attachments", attachments);
  }
  get xfaImages() {