Use archiving software like 7-Zip (Windows), The Unarchiver (Mac), or Keka (Mac).
Open only the first file ( a2860.7z.001 ) with your software, and it will automatically detect the other parts and extract the full content [1]. a2860.7z.007
If this file is part of a download you are managing, ensure you have downloaded all related parts, or extraction will fail. If you can tell me: Use archiving software like 7-Zip (Windows), The Unarchiver