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)


self.currentTask = 0;
          self._nextTask();
        }
      };
      if (this.delay > 0) {
        this._log("\nDelaying for " + this.delay + " ms...\n");
      }
      
      
      setTimeout(function () {
        r.send(null);
      }, this.delay);
    },
    _nextTask() {
      let failure = "";
      this._cleanup().then(() => {
        if (this.currentTask === this.manifest.length) {
          this._done();
          return;
        }
        const task = this.manifest[this.currentTask];
        task.round = 0;
        task.pageNum = task.firstPage || 1;
        task.stats = { times: [] };
        task.enableXfa = task.enableXfa === true;
        
        
        if (task.type === "other") {
          this._log(`Skipping file "${task.file}"\n`);
          if (!task.link) {