S02-nelly.7z

It is likely still in the process of downloading.

The suffix usually indicates that the file is an incomplete fragment of a larger download, often generated by download managers or BitTorrent clients (like Transmission) while a file is still being transferred. Once the download is 100% complete, the software typically removes the ".piece" extension to leave the final ".7z" archive. If you have this file on your computer: S02-Nelly.7z

If the download was interrupted, you'll need to resume it in the program that started it to get the full, usable file. It is likely still in the process of downloading

"S02-Nelly.7z" appears to be a specific archive file name, likely part of a split-volume set or a specific download package related to media or software. If you have this file on your computer:

Was this part of a specific or a media collection you were trying to download?

You generally cannot open a .piece file with extraction tools like 7-Zip until the rest of the data is present.