Duplicate uploaded file from dropzone or send again?

Question

AJAX

There are js dropzones, but no backend, but xhr is sent, the image is displayed as a preview.
Hung up id on input, does not react in any way to change, also tried through formdata - it displays empty data. Nothing is left in $_POST, or as now I get "Cannot read properties of undefined (reading '0')"

<form action="#" class="dropzone" id="js-form" enctype="multipart/form-data" method="POST">
                                                <div class="fallback">
                                                    <input name="file" id="js-file" type="file"
                                                        >
                                                </div>
                                                <div class="dz-message needsclick"><center>
                                                    <div class="mb-3">
                                                        <i class="display-4 text-muted bx
                                                            bx-cloud-upload"></i>
                                                    </div>

                                                    <h5>Перетащите или выберите для загрузки.</h5></center>
                                                </div>
                                            </form>
                                        </div>

                                        <div class="text-center mt-4">
                                            <button type="button" name="submit" id="upload_start_btn" class="btn btn-primary
                                                waves-effect waves-light">Загрузить</button>
                                        </div>

<script>
    Dropzone.autoDiscover = false;
    $(document).ready(function () {
        $(".dropzone").dropzone();
    });
</script>

<script>

    $('#upload_start_btn').on('click', function() {
    var file_data = $('#js-file').prop('files')[0];
    var form_data = new FormData();
    form_data.append('file', file_data);
    alert(form_data);
    $.ajax({
                url: 'ajax/file.php',
                dataType: 'text',
                cache: false,
                contentType: false,
                processData: false,
                data: form_data,
                type: 'post',
                success: function(php_script_response){
                    alert(php_script_response);
                }
     });
});

<?php

    if ( 0 < $_FILES['file']['error'] ) {
        echo 'Error: ' . $_FILES['file']['error'] . '<br>';
    }
    else {
        move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);
    }

?>

How to overcome and where to dig?