This seems obviously to be the right answer to me. Think about handling a ZIP file: you can't start at the beginning of the file and work forward from there. N

This seems obviously to be the right answer to me. Think about handling a ZIP file: you can't start at the beginning of the file and work forward from there. No; you have to start at/near the end of file and scan for an end-of-central-directory record based on a signature (it's not at a fixed offset due to a variable-length comment field). That then enables you to find the actual offset for the central directory itself; which in turns allow you to find the files themselves.