>But how do I prove I own an MP3 file?
It's not your job to prove you own something but their to prove you stole it.
That's a sound legal principle, but unfortunately it doesn't apply in all situations and in all jurisdictions:
"If the defendant is considered to have a criminal lifestyle, the court ... must decide whether he has benefited from his ‘general criminal conduct’ and, if so, by how much. ... The burden is on the defendant to rebut any assumption made."
https://www.rahmanravelli.co.uk/expertise/confiscation/confi...