Uploaded file not detected in POST

I've read a lot of similar issues but the solutions have not worked for me. I'd appreciate the help!

I have an index.php file which includes my HTML code and php code that moves the user's uploaded file to s3. My HTML form calls a js function sendEmails() which makes an AJAX request to another php script to validate the emails input and add it to a database. Everything is working except that the php code in my index.php file (at the very bottom) does not execute. It doesn't even go into the if statement and I don't understand why since it's supposed to execute when the user uploads a file and presses submit. If I put the $fileName = basename($_FILES["fileName"]["name"]) statements before the if statement then I get an undefined index error.

This is my HTML code in index.php:

<form action="javascript:void(0)" method="POST" id="files" enctype="multipart/form-data">
    <label class="col-md-4 col-form-label text-md-right">Select File:  <span class="text-danger">*</span></label>
    <input type="file" id="userFile" name="fileName" style="cursor: pointer; max-width: 170px;" onchange="enableBtn()">

    <label class="col-md-4 col-form-label text-md-right">Authorized Users:  <span class="text-danger">*</span></label>
    <input placeholder="Enter e-mail(s) here..." id="req" autocomplete="off"/>

    <button id="submitBtn" name="submitBtn" class="<?php echo SUBMIT_BUTTON_STYLE; ?>" onclick="return sendEmails()" disabled>Submit</button>
</form>

This is my php code in index.php:

<?php
$conn = new mysqli($servername, $username, $password, $db);
$sql = "SELECT sender_id, sender_email, receiver_emails, receiver_ids, file_name from filedrop_logs";
$result = mysqli_query($conn, $sql);

if ($result) {
    echo "<div class='outputDiv'>";
    echo "<table id='sharedOthers'>";
    echo "<thead><tr class='headings'>";
    echo "<th class='files'>Files</th>";
    echo "<th class='users'>Users</th>";
    echo "</tr></thead>";

    while ($row = mysqli_fetch_assoc($result)) {
        $receiverEmails = $row['receiver_emails'];
        $fileName = $row['file_name'];

        echo "<tbody id='bodyOthers'>";
        echo "<tr id='rowOthers'>";
        echo "<td>$fileName<br>";

        $objects = getListofObjects('FileDrop');
        foreach ($objects as $object) {
            $file = $object['Key'];
            $splits = explode('/', $file);
            if (end($splits) !== '') {
                $presignedUrl = getPresignedUrlForPrivateFile($object['Key'], '+20 minutes');
                $link = '<a href="'.$presignedUrl.'">Download</a>';
                echo $link;
            }
        }

        echo "&nbsp;&nbsp;&nbsp;&nbsp;<a href=''>Delete</a></td>";
        echo "<td>$receiverEmails</td>";
        echo "</tr></tbody>";
    }
    echo "</table></div>";
}
?>
<?php
//the if statement below doesn't execute

if(isset($_SERVER['REQUEST_METHOD']) && $_SERVER['REQUEST_METHOD'] === 'POST') {
$fileName = basename($_FILES["fileName"]["name"]);
$error = $_FILES["fileName"]["error"];
$tmpName = $_FILES["fileName"]["tmp_name"];

if (isset($_FILES["fileName"]) && $fileName != '' && $tmpName != '' && sys_get_temp_dir()) {
    $separator = DIRECTORY_SEPARATOR;
    $newDir = sys_get_temp_dir() . $separator . "FileDrop" . microtime(true);

    if (!file_exists($newDir)) {
        mkdir($newDir, 0777, true); // creates temp FileDrop directory
        $tempFilePath = $newDir . $separator . $fileName; // creates temp file inside FileDrop directory

        if (move_uploaded_file($tmpName, $tempFilePath)) { // moves file to tmp folder
            $s3FileName = "FileDrop" . substr($newDir, 4) . $separator . $fileName;
            $result = putFileToS3($s3FileName, $tempFilePath, 'public-read');
            deleteDir($newDir);
        }
    }
}
}
?>

This is my js code in case you want to see it:

function sendEmails() {
var fileData = $('#userFile').prop('files')[0];
var formData = new FormData();
formData.append('tags', JSON.stringify(tags));
formData.append('fileName', fileData);
$.ajax({
    type: "POST",
    url: "../FileDrop/dbSystem.php",
    processData: false,
    contentType: false,
    data: formData,
    success: function(result) {
        result = JSON.parse(result);
        if (result.validity === "valid emails") {
            location.reload();
            resetInputs(); //IMPORTANT
            $(".outputDiv").show();
        }
        else {
            var tagsBrackets = result.emails.toString().replace(/[\[\]']+/g,'');
            var tagsQuotes = tagsBrackets.replace(/['"]+/g, '');
            var tagsInvalid = tagsQuotes.replace(/,/g, ", ");
            $('#alertModal').modal({show:true});
            document.getElementById('invalid').textContent = tagsInvalid;
        }
    }
});
return false;
}

source https://stackoverflow.com/questions/67842488/uploaded-file-not-detected-in-post