Why is there an 'Attachment' option in some threads, but not others?

Specifically the medical conspiracies forum. I clicked the 'Quote' button and got to a reply form with an attachment option at the bottom.



I've only noticed it in the medical forum, and only when posting a reply to an existing thread. When I clicked the 'New Thread' button the form I get does not have this feature.

Questions:

1. What is it actually for? I mean what sort of attachments? PDF? JPEG? ZIP?
   
2. Where does the attachment go to, as in is it sent to the member upload space (therefore reducing the capacity in there) or is it somewhere else?
   
3. Why does it only appear in the medical forum? Why does it only appear when replying a thread for that matter?

Take your time, no rush answering this. I'm just curious about it is all.