Zip files appear as unknown format but still decompress
Zip files from my client are suddenly an "unknown" format but will decompress, usually. Sometimes it will take a couple of tries.
Mac mini (M1, 2020)
I would inquire of the client what archive tool they are using to zip their files.
Consider obtaining the free UnArchiver app from the Mac App Store. It is a "can opener" for many different compressed archive formats, some of which Apple makes no provision for opening. See if it improves on the decompression for your client's zip files.