Send an e-mail notification to custom user role when a file is uploaded to uploads folder

You could use the wp_mail function https://developer.wordpress.org/reference/functions/wp_mail/ to send an email to anyone after the file is successfully uploaded in your directory. You do not need any action here

Add it your code after the file is uploaded successfully

if(empty($errors)==true){
         move_uploaded_file($file_tmp,"/$user_dirname/".$document);
         echo "The file was uploaded successfully!";
         wp_mail(..);
      }else{
         print_r($errors);
      }

}