MindGenius map files since version 3.8 are XML-based (.mgmx). We decided to move to an XML-based file primarily to enable much simpler interaction with other applications.
MindGenius map files prior to version 3.8 used the old "MindGenius 2-3" format (.mgmf)
(Please see the help article on compatibility between version for more information)
XML, while originally intended for marking up electronic documents is now commonly used to represent any kind of data structure, for example application file types. A great many applications now read and write XML-based files because they're much easier to read (human readable even), validate for acceptable data and develop due to their conformance to a defined schema.
The move to XML has also brought a significant reduction in file size for your average map, which is beneficial to MindGenius users, particularly for those making use of the mobile exports feature where file size is important.
Saving an old style .mgmf map in the newer .mgmx format