/*
 * $file is the path to your uploaded file (for example as set in the $_FILE posted file array)
 * $filename is the name of the file
 * first we need to upload the file into the wp upload folder.
 */
$upload_file = wp_upload_bits( $filename, null, @file_get_contents( $file ) );
i
f ( ! $upload_file['error'] ) {
  // if succesfull insert the new file into the media library (create a new attachment post type).
  $wp_filetype = wp_check_filetype($filename, null );

  $attachment = array(
    'post_mime_type' => $wp_filetype['type'],
	'post_parent'    => $post_id,
	'post_title'     => preg_replace( '/\.[^.]+$/', '', $filename ),
	'post_content'   => '',
	'post_status'    => 'inherit'
  );

  $attachment_id = wp_insert_attachment( $attachment, $upload_file['file'], $post_id );

  if ( ! is_wp_error( $attachment_id ) ) {
     // if attachment post was successfully created, insert it as a thumbnail to the post $post_id.
     require_once(ABSPATH . "wp-admin" . '/includes/image.php');

     $attachment_data = wp_generate_attachment_metadata( $attachment_id, $upload_file['file'] );

     wp_update_attachment_metadata( $attachment_id,  $attachment_data );
     set_post_thumbnail( $post_id, $attachment_id );
   }
}