You will likely need to use a combination of the Web Request action and Vadim's custom Base64 Workflow action (http://workflowexcellence.com/nintex-workflow-get-list-item-attachment-in-base64-custom-action-happy-thanksgiving/) in order to do what you'd like to do. 

As of now, this is the closest thing I've found that explains it, but it is taking attachments from a task and then attaching them to a different task. In your case you will simply attach them to a list item or put them into a document library elsewhere, but that should make little difference: https://community.nintex.com/t5/Community-blogs/collect-task-attachments-in-list-item-and-show-all-attachments/ba-p/82470 

If you have further questions, please ask. 

Honestly setting up the custom part went straight over my head and I was unable to figure this out so I went about a different way of pulling the attachment to make it viewable.

I ended up using a request data task and stored the link in a workflow variable that I was able to reference later in a request review task where users could click the link to open the attachment instead.

Thank you for your help!

No problem. It's not straight forward at all! Glad to hear that you came up with a solution though! 

Please mark your comment as the Solution at your leisure.