diff --git a/booking.email_manually.inc b/booking.email_manually.inc
index c237414..afc3cab 100644
--- a/booking.email_manually.inc
+++ b/booking.email_manually.inc
@@ -129,7 +129,7 @@ function booking_manual_email_form($form, &$form_state)
   $form['booking-email-attachment'] = array(
     '#type' => 'managed_file',
 		'#title' => t('Email attachment'),
-		'#description' => t('Attach a file to be emailed. Note that this only works with custom email types.'),
+		'#description' => t('Attach a file to be emailed. Note that this only works with custom email types. File extension must be one of: ' . variable_get('booking_email_allowed_attachments', 'pdf docx')),
     '#upload_location' => 'public://bookings/emails',
     '#upload_validators' => array(variable_get('booking_email_allowed_attachments', 'pdf docx')),
   );