Ask the ISPs how much data they've uploaded
If the damages awarded to the prosecution are related to how many copies were illegally distributed, why not do the following:
Require the prosecution to supply the ISP's record of how much data the defendant uploaded during the time they were sharing the file?
I'm pretty certain ISPs collect this data anyway to help them manage their networks more effectively. If the prosecution is forced to include this data in any claim, it puts an upper limit on how much they can claim in damages.
So if the file in question is 20MB and your ISP's logs show that you uploaded 200MB over the few days you were sharing it, the maximum possible damages that could be awarded to the prosecution would be for 10 illegal copies.