Attach (recommended) or Link to PDF file here:
issue13500 (in test corpus)

This regressed when landing #13566. 

What is the expected behavior? (add screenshot)
<img width="664" alt="image" src="https://user-images.githubusercontent.com/942640/122467522-7bc8aa80-cf6f-11eb-9d4d-311eb7c64fb2.png">


What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/942640/122467565-85eaa900-cf6f-11eb-9c10-8a8686ed440c.png)


constructor(attributes) {
    super(TEMPLATE_NS_ID, "submit",  true);
    this.embedPDF = getInteger({
      data: attributes.embedPDF,
      defaultValue: 0,
      validate: x => x === 1,
    });
    this.format = getStringOption(attributes.format, [
      "xdp",
      "formdata",
      "pdf",
      "urlencoded",
      "xfd",
      "xml",
    ]);
    this.id = attributes.id || "";
    this.target = attributes.target || "";
    this.textEncoding = getKeyword({
      data: attributes.textEncoding
        ? attributes.textEncoding.toLowerCase()
        : "",
      defaultValue: "",
      validate: k =>
        [
          "utf-8",
          "big-five",
          "fontspecific",
          "gbk",
          "gb-18030",
          "gb-2312",