David,
Actually, you don't have to provide them with your email address. They will see your email address as the "from" address on the private message you send them and can just reply to your message. WikiTree only protects their email address from being shown to anyone who decides to write to them.
The WikiTree private message system is not sufficiently robust to include ability to send file attachments, but you can tell the person that you'd like to send them a file if they reply to your message so that you will have their email address.