a) that the file being offered was in fact the copyrighted work
b) that you had all or a substantial portion of it
c) that you were offering it for upload
This information is easily available to them, if they recorded it correctly and convince the judge that their techniques are valid and accurate.
Whether you were uploading or not, making an unauthorized copy is the basic illegal act. Offering it for upload just increases the likelihood of a higher fine from the court, and opens up the remote possibility that you could be sued for actual damages if there were a way for the complainant to prove lost sales due to your unauthorized copy being offered on the internet.