Hi Daniel,
Great question! Event Tickets Plus does indeed allow you to create your own registration fields directly in the plugin. Unfortunately, attached files is not one of the available fields in the feature.
I do think you could use something like Contact Form 7 or Gravity Forms to capture attendee data, but the trick (and where there would likely be a fair amount of custom development) is attaching that data from the form plugin to the tickets that are sold with each ticket. For example, if multiple tickets are sold in a single transaction, then there would have to be some way for the form plugin to relate that information back to the specific tickets that were sold on a ticket-by-ticket basis.
Does this help answer your question? Please let know if you have any other questions and I’d be happy to help as best I can!
Cheers,
Geoff