Upload file to remote storage

Two options:

1. Hook into 'wp_handle_upload', a filter provided by the function wp_handle_upload() in wp-admin/includes/file.php:

   apply_filters( 
       'wp_handle_upload', 
       array( 'file' => $new_file, 'url' => $url, 'type' => $type ), 'upload' 
   )
   

   Replace the new file URI with your remote URI.

2. The function wp_insert_attachment() in wp-includes/post.php offers two actions:

   do_action('edit_attachment', $post_ID);
   do_action('add_attachment', $post_ID);
   

   You can get the attachment data by $post_ID and change any value here.