Change attachment filename

This will allow you to rename an attachment as soon as its uploaded:

add_action('add_attachment', 'rename_attachment');
function rename_attachment($post_ID){

    $file = get_attached_file($post_ID);
    $path = pathinfo($file);
        //dirname   = File Path
        //basename  = Filename.Extension
        //extension = Extension
        //filename  = Filename

    $newfilename = "NEW FILE NAME HERE";
    $newfile = $path['dirname']."https://wordpress.stackexchange.com/".$newfilename.".".$path['extension'];

    rename($file, $newfile);    
    update_attached_file( $post_ID, $newfile );

}