There is only one file type that uses the .MVDX extension:
MindView Windows Document
Document created and used by MindView, a mind mapping program used to visually brainstorm and organize ideas; contains one or more ideas made up of text, shapes, images, branches, sub-branches, and other mind map elements.
NOTE: The MVDX file can only be saved by the Windows version of MindView, but it can be opened by the OS X version of MindView. The MindView for OS X saves documents with the .MVD file extension.
Updated: June 20, 2015