OK, let's see. No, I don't know of a modification exactly like that.
I do know of modifications similar to that used on trackers.
Don't read this part if you don't plan on writing the code yourself:
If I were to need a modification like this (which I don't), I would go to a tracker site and get the code for 1) uploading the nfo and grabbing the content 2) spitting the file content out into the page. Then, I would look at the attachment.php file and find a good hook location and then add the code I just got from the tracker site (for uploading the nfo and grabbing the content) into a plugin using that hook location. Then, I would look at the showthread.php page and find another good hook location and write a plugin using that hook to do the second thing - spit the file content out into the page. That is simply the process I would go through if this were a project I was taking on.
If you don't want to write the code yourself, then you might want to consider hiring someone to do it for you in the
Requests For Paid Services or wait and see if someone else feels like tackling this project.
edit: if I recall, you just need to use a fixed width font to spit out the file content like Courier New (I think that's the one we use). I just tested nfo extension on my test site and they load just fine so all you need to do is hook into an area in showthread that has to do with the attachment and write some code dealing with displaying an nfo file (just like vb has code dealing with displaying jpg or gif files).