Custom image uploader without widgets

Ended up doing it like this:

In functions.php:

//SEND IMAGE INTO WORDPRESS

function new_attachment(){

//echo 'talking to new_attachement in functions.php';

if(

! wp_verify_nonce( $_POST['client-file-upload'], 'add_attachement' ) //adding a layer of security by verifying a nonce that was set in the addrecipe.php form field
){

    wp_nonce_ays( '' ); //Display “Are You Sure” message to confirm the action being taken. A = are Y = You S = Sure. Don't know if needed here.
}

require_once(ABSPATH . 'wp-admin/includes/admin.php');
$myfiles = $_POST["async-upload"];
$id = media_handle_upload('async-upload', 0); //post id of Client Files page
$attachment_url = wp_get_attachment_url($id);
unset($myfiles);


if ( is_wp_error($id) ) {
    $errors['upload_error'] = $id;
    $id = false;
}

if ($errors) {
    echo "<p>There was an error uploading your file.</p>";
} else {
    echo  $attachment_url;
}

exit();




}
add_action('wp_ajax_add_attachement','new_attachment'); //add an image file 

Javascript:

    jQuery('#file-form').submit(
    function (e) {
        e.preventDefault();
        let mydata = new FormData(this);
        mydata.append('action', 'add_attachement');
        jQuery.ajax({
            url: adminAjaxURL,
            type: 'POST',
            data: mydata,
            processData: false,
            contentType: false,
            success: function (result) {
                console.log(result);
                jQuery('#imageURLs').val(result);
                //$("#div1").html(str);
            }
        })
    });

PHP and HTML:

<form id="file-form" enctype="multipart/form-data">
    <p id="async-upload-wrap">
    <label for="async-upload">Lataa kuva</label>
    <input type="file" id="async-upload" name="async-upload"> <input type="submit" value="Lataa" name="html-upload">
    </p>

    <p>
    <input type="hidden" name="post_id" id="post_id" value="<?php echo '0';?>" />
    <?php wp_nonce_field('add_attachement', 'client-file-upload'); ?>
    </p>

    <p>
    <input type="submit" value="Save all changes" name="save" style="display: none;">
    </p>
    </form>