很多类型的文件，其起始的几个字节的内容是固定的（或是有意填充，或是本就如此）。
根据这几个字节的内容就可以确定文件类型，因此这几个字节的内容被称为魔数 (magic number)。
JPEG jpg;jpeg 0xFFD8FF PNG png 0x89504E470D0A1A0A GIF gif GIF8 TIFF tif;tiff 0x49492A00 TIFF tif;tiff 0x4D4D002A AOL ART art 0x4A47040E000000 AOL ART art 0x4A47030E000000 PC Paintbrush pcx 0x0A050108 Graphics Metafile wmf 0xD7CDC69A Graphics Metafile wmf 0x01000900 Graphics Metafile wmf 0x02000900 Enhanced Metafile emf 0x0100000058000000 CAD dwg 0x41433130 Outlook Express dbx 0xCFAD12FE MS Office/OLE2 doc;xls;dot;ppt;xla;ppa;pps;pot;msi;sdw;db 0xD0CF11E0A1B11AE1 WordPerfect wpd 0xFF575043 Quicken qdf 0xAC9EBD8F QuickBooks Backup qbb 0x458600000600 Sage sly.or.srt.or.slt;sly;srt;slt 0x53520100 Lotus WordPro v9 lwp 0x576F726450726F Lotus 123 v9 123 0x00001A00051004 Lotus 123 v5 wk4 0x00001A0002100400 Lotus 123 v3 wk3 0x00001A0000100400 Lotus 123 v1 wk1 0x2000604060 Windows Password pwl 0xE3828596 ZIP Archive zip;jar 0x504B0304 ZIP Archive (outdated) zip 0x504B3030 GZ Archive gz;tgz 0x1F8B08 ARJ Archive arj 0x60EA MPEG mpg;mpeg 0x000001BA MPEG mpg;mpeg 0x000001B3 Windows Media asf 0x3026B2758E66CF11 ELF Executable elf;; 0x7F454C4601010100 Java-Class class 0xCAFEBASE