Attach (recommended) or Link to PDF file here:
https://guichet.public.lu/dam-assets/catalogue-formulaires/tva/tva-option-aic-pers-morales/tva-option-aic-pers-morale-DE.pdf

Configuration:
- Web browser and its version: Firefox Nightly 91.0a1 (2021-06-26) (64-bit)
- Operating system and its version: Windows 10.0.19043.1052
- PDF.js version: N/A

Steps to reproduce the problem:
1. enableXFA in Firefox Nightly
2. Open form

What is the expected behavior? (add screenshot)
Header image is scaled properly
![image](https://user-images.githubusercontent.com/7104454/123525954-905d1f00-d6a2-11eb-840e-be30eee395e1.png)

What went wrong? (add screenshot)
Image is scaled to fit
![image](https://user-images.githubusercontent.com/7104454/123525957-96530000-d6a2-11eb-9991-bf7f37c6f1a5.png)


const style = toStyle(this, "font", "margin", "para", "visibility");
    switch (this.placement) {
      case "left":
      case "right":
        if (this.reserve > 0) {
          style.width = measureToString(this.reserve);
        } else {
          style.minWidth = measureToString(this.reserve);
        }
        break;
      case "top":
      case "bottom":
        if (this.reserve > 0) {
          style.height = measureToString(this.reserve);
        } else {
          style.minHeight = measureToString(this.reserve);
        }
        break;
    }
    return HTMLResult.success({
      name: "div",
      attributes: {
        style,
        class: ["xfaCaption"],
      },
      children,
    });
  }
}
class Certificate extends StringObject {