Please refer my previous article to learn more about creating NINTEX forms in SharePoint 2013 Online.
I have created a custom NINTEX form in the custom list named Custom List which has a Title and Attachment field. In this blog you will see how to set attachment validations in NINTEX forms.
Navigate to the Custom List. Click on List tab and then click on NINTEX Forms button in the ribbon interface. Right click on Attachment control and then click on Settings.
Set the attachment validation as shown below and click on Save button. Click on Publish and Close in the ribbon interface.
Add a new item and when you try to save the item without attaching a document.
When you try to add an attachment with other formats except doc or docx.
Add document link is disabled after a document is attached.
Thus in this blog you have seen how to set attachment validations in SharePoint 2013 Online NINTEX forms.