PHP error when trying to use wpcf7_before_send_mail

Hi,

I’m looking to modify the form output before it sends the email using the base plugin hook “wpcf7_before_send_mail” to dynamically add a file attachment. I’ve created the following function:

do_action( 'wpcf7_before_send_mail',  'my_function'); 

function my_function($wpcf7) { 
  echo 'test';
}

and I receive the following error:

Fatal error: Uncaught ArgumentCountError: Too few arguments to function CF7CF::hide_hidden_mail_fields(), 1 passed in /home/fifteeni/test.domainname.co.uk/wp-includes/class-wp-hook.php on line 287 and exactly 3 expected in /home/domainname/test.domainname.co.uk/wp-content/plugins/cf7-conditional-fields/cf7cf.php:239

I’ve tried adding two more parameters to the function call to make sure I capture all of the incoming arguments but I receive the same error.

Can anyone please help?