Sending Uploaded Files Using HTML Form Through Javascript to API

Question

How can i sending uploaded files using HTML form through Javascript to API? Below was the sample code. I'm using web-to-lead method by EspoCRM.

<div id="web-to-lead-form-container">
    <form id="web-to-lead-form">
        <div>
            <input type="text" name="firstName" placeholder="First Name">
        </div>
        <div>
            <input type="text" name="lastName" placeholder="Last Name" required>
        </div>
        <div>
            <input type="email" name="emailAddress" placeholder="Email Address" required>
        </div>
        <div>
            <input type="file" name="firstDocument">
        </div>
        <div>
            <input type="file" name="secondDocument">
        </div>
        <div>
            <button type="submit" name="submit">Submit</button>
        </div>
    </form>
</div>

<script type="text/javascript">
    let webToLeadFormElement = document.getElementById('web-to-lead-form');
    let webToLeadFormIsSubmitted = false;

    webToLeadFormElement.addEventListener('submit', event => {
        event.preventDefault();

        if (webToLeadFormIsSubmitted) {
            return;
        }

        webToLeadFormIsSubmitted = true;
        webToLeadFormElement.submit.setAttribute('disabled', 'disabled');

        let payloadData = {
            firstName: webToLeadFormElement.firstName.value || null,
            lastName: webToLeadFormElement.lastName.value || null,
            emailAddress: webToLeadFormElement.emailAddress.value || null,
        };

        // A needed URL can be found on the Lead Capture detail view.
        let url = 'https://URL_OF_YOUR_CRM/api/v1/LeadCapture/API_KEY';

        let xhr = new XMLHttpRequest();

        xhr.open('POST', url, true);

        xhr.setRequestHeader('Content-Type', 'application/json');
        xhr.setRequestHeader('Accept', 'application/json');

        xhr.onreadystatechange = () => {
            if (this.readyState == XMLHttpRequest.DONE && this.status == 200) {
                let containerElement = document.getElementById('web-to-lead-form-container');

                containerElement.innerHTML = 'Sent';
            }
        };

        xhr.onerror = () => {
            webToLeadFormElement.submit.removeAttribute('disabled');
            webToLeadFormIsSubmitted = false;
        };

        xhr.send(JSON.stringify(payloadData));
    });
</script>

I try to put firstDocument: webToLeadFormElement.firstDocument.value || null, on letpayloadData but nothing happen. I know its wrong because im still new on javascript.