We sometimes have files with binary data, i.e., where you convert int to binary then encode it into the file — this is generally a pretty bad idea, because int and float numbers are encoded differently on different systems.
The exceptions are .jpeg and .mpeg. To IO binary, you need to use write().
Magic numbers
One key aspect of file metadata is the magic number, which is used to identify the file format of a given file. These consist of several carefully encoded bytes at the beginning of the file, distinct for the file format.