File extension
The method is simple, unsafe
Binary file header
Magic number (as programmed in the magic number), referred to herein as the first four bytes of two file mechanisms. Different file types, four different bytes Content
For .class files, also followed behind the magic number used java version information
Of course, some of the magic number of the file (the first four bytes) content is not fixed, it needs (two mechanisms contents of the intermediate file or the contents of two mechanisms end of a location, etc.) to identify other ways
Use software to identify: File command
How to understand and identify the types of files in Linux