Issue
In Nintex Forms, attachment control may not be validated for name length and special characters before submitting form.
Resolution
Upgrade to Nintex Form 2013 hotfix version 2.11.3.20 or Nintex Forms July 2018 release.
Upgrade to Nintex Form 2016 hotfix version 4.3.2.20 or Nintex Forms July 2018 release.