This extension has been deprecated as the PECL extension Fileinfo provides the same functionality (and more) in a much cleaner way.
The functions in this module try to guess the content type and encoding of a file by looking for certain magic byte sequences at specific positions within the file. While this is not a bullet proof approach the heuristics used do a very good job.
This extension is derived from Apache mod_mime_magic, which is itself based on the file command maintained by Ian F. Darwin. See the source code for further historic and copyright information.