I can give you exact steps or a small script for your case.
If you need to convert dozens of ZIP files to SB3 (e.g., restoring a classroom server backup), use this batch rename: zip to sb3
This usually happens when you create the ZIP manually. Use an actual Scratch project (exported from the editor) as a template. Compare your ZIP’s internal hierarchy to a known working SB3 (renamed to ZIP). I can give you exact steps or a small script for your case