Setting featured image from front end form - php

Question

So I have been trawling the internet for a few days looking for a solution to this problem. I am trying to set the featured image of a post from the upload input of this form:
<form method="post" action="" id="quick-post-form" enctype="multipart/form-data">
<textarea id="input-box" placeholder="What's on your mind?" maxlength="10000" name="quick-post-area" class="col-xs-12"></textarea>
<!-- images -->
<fieldset class="images">
<label for="images">Upload an image</label>
<input type="file" name="image-uploader" id="img-upload" size="50">
</fieldset>
<!-- images end -->
<input type="submit" value="Post" name="submit" class="col-md-4 col-xs-12" id="quick-post-submit">
</form>
I am using wp insert post to create a post from this form but the image is not uploaded as it should be. This is the PHP I'm using:
<?php
function quick_post() {
// Initialize the page ID to -1. This indicates no action has been taken.
$post_id = -1;
$author_id = 1;
$slug = 'post';
global $current_user;
$user_id = get_current_user_id();
// If the page doesn't already exist, then create it
if( null == get_page_by_title( $title ) ) {
$post_id = wp_insert_post(
array(
'comment_status' => 'open',
'ping_status' => 'closed',
'post_author' => $user_id,
'post_name' => $slug,
'post_title' => 'Post By: ' . $current_user->user_login,
'post_status' => 'publish',
'post_type' => 'post',
'post_content' => $_POST['quick-post-area'],
'post_category' => array( 3 ),
)
);
require_once (ABSPATH.'/wp-admin/includes/media.php');
require_once (ABSPATH.'/wp-admin/includes/file.php');
require_once (ABSPATH.'/wp-admin/includes/image.php');
$attachmentId = media_handle_upload('image-uploader', $post_ID);
set_post_thumbnail($post_ID, $attachmentId);
// Otherwise, we'll stop
} else {
// Arbitrarily use -2 to indicate that the page with the title already exists
$post_id = -2;
} // end if
} // end programmatically_create_post
if(isset($_POST['submit']))
{
quick_post();
header('Location: https://outrankd.com');
}
?>
I should also mention that once this is working, I plan to use the Wordpress check filetype function to validate the upload but think that this should work with something like this:
if ($attachmentId = wp_check_filetype('image')) {
set_post_thumbnail($post_ID, $attachmentId);
}
If anyone could shed some light on where I'm going wrong, it would be much appreciated.
Thanks