So, since the Attachments in Nintex Forms try to download rather than open in Office Online, and that doesn't appear to be fixable/customizable, I'm working on a workaround.
Leveraging both this SlideShare from Alan Richards and this great blogpost: Attachment links in 'Send e-mail' I am able to successfully generate a link to the attachment which opens in Office Online Apps! Great! But now, they're ugly on the form. If I try to set them into a "Hyperlink" field, I can't set the "display text" and I'd have to have enough link fields for each attachment which, depending on how many, could be a really ugly and terrible deal. If I set them into a text field of any type (rich or not) it just puts the link, not clickable, and again, I have no way to make it pretty.
I tried to mess with the Calculated Field to see if I could get something going, but couldn't quite get there. Any tips or suggestions?