Problem with fsPromises.readFile used in module

Question

I use XMLHttpRequest in Node.js and when handling request in server.js (asynchronous POST request) I use this code:

      const filePath = './input.pdf';
      fsPromises.readFile(filePath)
        .then(res => {
          const pdfBytes = new Uint8Array(res).buffer;
        })
        .catch(err => {
          console.log('Could not read file-template');
        });

then I get ArrayBuffer through this code. But I want to move this code into module and call export function there with this code. This change disrupted reading file and devtools network returns error in my request with message 'Provisional headers are shown'.

How can I make it work in module, not in server.js?

I attached images with messages in devtools img1, img2