Hi. The attachments were, in my case, photocopies of my CAA license and medical. These are not mandatory (they check you out with the CAA anyway) but they ask for them if possible and I assume it smooths the process a bit.
I posted mine, rather than faxing - I don't think there was a fax option at the time. Even so, I felt happier sending the actual bits of paper. They arrived 3 days later (got an Email confirmation).