Files have signatures or 'magic numbers' embedded in them, usually near the beginning of the file. Libmagic is a library which extracts a files signature and looks it up in a signature database. This is the way Unix type systems determine file types i.e.

How Do You Use A Software Patch. If you save a text file without an extension on Linux it will still automatically open with a text editor. Systems like Windows on the other hand only look at file extension. Opening a text file with no extension on Windows will result in a WTf-is-this popup window. So there are merits in checking both the extension and the magic number since your website will likely have visitors with different operation systems. True, although I would add that not all files have magic numbers. In particular, Linux-like systems tend to recognize a text file by its lack of a known magic number. And depending on which program is doing the interpretation, a known file extension could override the type determination from the magic number. Queried Update Status Missing Sccm more.