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You generally cannot open a .piece file with extraction tools like 7-Zip until the rest of the data is present.
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
"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. You generally cannot open a
Was this part of a specific or a media collection you were trying to download? S02-Nelly.7z