have a look here how I overcame problems with attachments (and several other problems)

May Mission - Universal task form

regarding adding/deleting attachments, the best practice is to keep attachments just on list items and do not add them to tasks (task list items). that way if a 'task is returned' back to initiator, he/she can freely update his/her own list item, including attachments (Note you do not in fact return the task itself but rather just not approve original item's content. initiator should get assigned new task to do corrections)

Thanks Marian Hatala​ for your response. I read your article, but i don't think it fully addresses the requirement i have from my users, or maybe I didn't understand that part. Please clarify.

How would an initiator update attachments? I know its fairly straightforward to update any other list column but attachments.

initiator has to open list item in edit mode/form.

there, apart from links of already attached documents, he should see 'Delete' and 'Add Attachment' links (of course, if you haven't configured the control not to provide them)

The problem with this is that the user has to go to the list, look for the item, select it and click Edit Item.

It would be nice if i could have the item edit form within a task (initiator revise)

as I have mentioned in the document, below the comment field there is direct link to item's edit form.

so it's just one simple click

Oh i missed that part. I think your solution will save me all the headache and resolve my problem. But i still don't get how you included that link to the edit form. Is it a Nintex built in reference or it's something you configured yourself?

it's not provided OOTB by nintex, but you can easily build it on your own.

nintex provides 'Item URL' reference what's in fact link to  item's display form. to open item's edit form, following do the trick

fn-Replace(Item URL ,"DispForm","EditForm")