7z

One 7z archive holding three files, 230 bytes in total. 7z is the format most often missing from file tools, since the standard libraries in many languages cannot open it without an extra package. If your application unpacks archives, this file tells you quickly whether 7z is supported or whether it fails with an unhelpful error. It uses the LZMA family of compression, so a tool that only handles zip-style deflate will not read it. Check that your code detects the format from the file contents, that it lists the three entries, and that a clear message appears if 7z is not allowed.