The .zip file and .xlsx/.docx is a good example of the complexity. It's popular to build certain application files (word docs, excel spreadsheets) that are zip

The .zip file and .xlsx/.docx is a good example of the complexity. It's popular to build certain application files (word docs, excel spreadsheets) that are zip files on the outside. Something with the first bytes 0x50 0x4d 0x03 0x04 (iirc) could be a zip file that the user wants to unzip or an application file that the user wants to open with an application. You need to probe deeper to guess which it is, or rely on the file extension.