Is there a file upload field that I can add to my booking form?
As at October 2018, there is no such field. However, there is a workaround.
You can get your users to simply upload their files to dropbox, google drive or such services and simply paste the link to that file in your booking form.
Though this field is not difficult to setup, it is not simple from a legal perspective as it would involve us storing potentially sensitive information.