ZIM is a file format for storing complete dumps of wikis so that they are usable for offline reading, with the entire contents contained in a single, relatively compact file. It is based on the older Zeno format which is no longer in use.
The first four bytes are (hex)
5a 49 4d 04, which represents the long integer "magic number" 72173914 (decimal) in little-endian form. The first three bytes also spell "ZIM" in ASCII. The two bytes after the first four represent the major and minor version numbers of the file format version.