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<?php
/* Dynamically add file attachments to Contact Form 7 emails from a Wordpress custom field.
 * Custom field is 'case-pdf' in the example.
 */

add_action('wpcf7_before_send_mail', 'wpcf7_add_attachment');

function wpcf7_add_attachment($contact_form) {
  global $_POST;
  $submission = WPCF7_Submission::get_instance();

  if ( $submission && !empty($_POST['case-pdf']) ) {
    $mail = $contact_form->prop('mail');
    $mail['attachments'] .= $_POST['case-pdf'];
    $contact_form->set_properties(array('mail' => $mail));
  }
}