Post form - AJAX won't upload featured image - Plugin development

Question

I have a form for adding a post via AJAX and PHP. I have three fields. Text, textarea and file input field.

Everything it's working, but it won't upload a featured image. When I remove the AJAX call, then it's working. It only uploads the text and the textarea field.

Here is the form:

public function pp_html_template() {

    if ( is_user_logged_in() ) {
        return '<h2>' . __( 'Publish your post', 'post-publisher' ) . '</h2>
        <form id="pp-form-submit" name="pp-form-submit" class="pp-form-submit" enctype="multipart/form-data">' .
       wp_nonce_field( 'pp_publisher_save', 'pp_publisher_name' )
       . '<div class="pp-row">
                <label for="pp_title">' . esc_attr__( 'Title', 'post-publisher' ) . '</label>
                <input type="text" id="pp_title" name="pp_title">
            </div>

            <div class="pp-row">
                <label for="pp_content">' . esc_attr__( 'Content', 'post-publisher' ) . '</label>
                <textarea name="pp_content" id="pp_content" name="pp_content" cols="30" rows="10"></textarea>
            </div>
            
            <div class="pp-row">
                <label for="pp_featured_image">' . esc_attr__( 'Featured Image', 'post-publisher' ) . '</label>
                <input type="file" id="pp_featured_image" name="pp_featured_image">
            </div>
            <input type="hidden" name="action" value="pp_html_process"/>
            <div class="pp-row">
                <input type="submit" name="pp_submit" id="pp_submit">
            </div>
        </form>';
    }
}

Here is the form process:

public function pp_html_process() {

    // Process the form
    if ( isset( $_POST['pp_submit'] ) ) {
        if ( ! isset( $_POST['pp_publisher_name'] ) || ! wp_verify_nonce( $_POST['pp_publisher_name'], 'pp_publisher_save' ) ) {
            esc_attr__( 'Sorry, this action is not allowed.', 'post-publisher' );
            exit;
        } else {
            global $current_user;

            $user_login   = $current_user->user_login;
            $user_id      = $current_user->ID;
            $post_title   = sanitize_text_field( $_POST['pp_title'] );
            $post_content = sanitize_textarea_field( $_POST['pp_content'] );

            $new_post = array(
                'post_title'   => $post_title,
                'post_content' => $post_content,
                'post_type'    => 'post',
                'post_status'  => 'draft',
                'post_name'    => str_replace( ' ', '-', $post_title ),
            );

            $post_id = wp_insert_post( $new_post, true );

            if ( ! function_exists( 'wp_generate_attachment_metadata' ) ) {
                require_once( ABSPATH . "wp-admin" . '/includes/image.php' );
                require_once( ABSPATH . "wp-admin" . '/includes/file.php' );
                require_once( ABSPATH . "wp-admin" . '/includes/media.php' );
            }


            $featured_image = media_handle_upload( 'pp_featured_image', $post_id );
            if ( $featured_image > 0 ) {
                update_post_meta( $post_id, '_thumbnail_id', $featured_image );
            }

            if ( wp_doing_ajax() ) {
                echo 'Post created! ID';
                wp_die();
            }
        }
    }
}

Here is the __contstruct()

public function __construct() {
    if ( ! is_admin() ) {
        add_shortcode( 'pp_html_template', array( $this, 'pp_html_template' ) );
        add_action( 'template_redirect', array( $this, 'pp_html_process' ) );
    }
    add_action( 'wp_ajax_pp_html_process', array( $this, 'pp_html_process' ) );
    add_action( 'wp_ajax_nopriv_pp_html_process', array( $this, 'pp_html_process' ) );

}

And here is the AJAX:

jQuery(document).ready(function ($) {
    function ajaxSubmit() {
        var formData = new FormData(this);

        formData.append('pp_submit', 1);

        $.ajax({
            action: 'pp_featured_image',
            type: 'POST',
            url: public_ajax.ajaxurl,
            data: formData,
            processData: false,
            contentType: false,
            success: function (response) {
                console.log('success', response);
            },
            error: function (response) {
                console.log('error!', response);
            }
        });

        return false;
    }

    $('#pp-form-submit').submit(ajaxSubmit);
});

I'm not getting any errors in the console. Any help would be appreciated.