file myfile.arc If it shows Zip archive data , it’s actually a ZIP file — rename to .zip and use standard unzip tools.
The error message typically appears when trying to open a file with FreeArc ( .arc extension) or an archiver that relies on its engine (like some versions of PeaZip, or when using the arc command-line tool). file myfile
Look at the first few bytes (magic bytes): file myfile