constructor(root, xref, skipContent = false) {
    if (!(root instanceof Dict)) {
      return;
    }
    this.xref = xref;
    this.root = root;
    if (root.has("FS")) {
      this.fs = root.get("FS");
    }
    if (root.has("RF")) {
      warn("Related file specifications are not supported");
    }
    if (!skipContent) {
      if (root.has("EF")) {
        this.#contentAvailable = true;
      } else {
        warn("Non-embedded file specifications are not supported");
      }
    }
  }
"A PDF file can refer to the contents of another file by using a File
Specification (PDF 1.1)", see the spec (7.11) for more details.
NOTE: Only embedded files are supported (as part of the attachments support)
TODO: support the 'URL' file system (with caching if !/V), portable
collections attributes and related files (/RF)