you can build links to item attachments on your own within workflow and then show them on task form.

have a look on this https://community.nintex.com/message/48946-re-attachment-url-from-a-list-item-to-task-itemapproval-form?commentID=48946#… 

Hi Marian, Thank you for your reply. As I understand this solution works only for SharePoint default task form. I'm using Nintex task forms to do that. 

it should work with nintex task form.

prepare link(s) to a workflow variables and then present them somehow on the form (eg. in calc. value control, reach text control, ...)