Cant move file after media_handle_upload

Question

You’re doing it twice – media_handle_upload is a wrapper for wp_handle_upload.

Just use the former like so:

require_once ABSPATH . 'wp-admin/includes/file.php';
$id = media_handle_upload(
    'custom_upload',
    $post->ID,
    array(), // Attachment post data overrides, WordPress will do all the grunt work by default
    array(
        'test_form' => false,
        'action' => 'editpost',
    )
);

if ( ! is_wp_error( $id ) )
    update_post_meta( $post->ID, 'custom_upload', $id );