There is only one file type that uses the .MGMX extension:
MindGenius XML Map File
Mind map created by MindGenius business diagramming software; saves a diagram such as a brainstorming chart, project plan, or a flowchart; used for storing presentation materials as well as for creating business documentation; can be exported to a variety of document formats, including PowerPoint (.PPTX), Visio (.VSD), and .PDF formats.
The MGMX format uses XML formatting and was introduced with MindGenius 4. MindGenius 2-3 use the .MGMF format, which uses binary formatting instead of XML formatting.
Updated: August 1, 2011