unable to set uploaded file via frontend form as featured image

first of all you need to add enctype=”multipart/form-data” to make your form support file uploads.

So far you’ve attached the image to the post, but you haven’t inserted them. Try something like this

<?php 
$args = array( 'post_type' => 'attachment', 'numberposts' => -1, 'post_status' =>   null, 'post_parent' => $post->ID );  
$attachments = get_posts($args);  
if ($attachments) { 
    foreach ( $attachments as $attachment ) { ?>

    <?php echo apply_filters( 'the_title' , $attachment->post_title ); ?>

    <?php the_attachment_link( $attachment->ID , false ); ?>

<?php }  
}
?>