There is only one file type that uses the .MDJ extension:
StarUML Model File
Model file created and used by StarUML, a software modeling application; contains project information, which includes the project type and classes added to the project diagram; stored in JSON format.
StarUML uses the Unified Modeling Language (UML) to visualize the design of software. When you create a new project or load a saved project, you are using the model file. As a precaution, StarUML makes a backup of your working model file every 5 minutes and if the application quits unexpectedly, the backup will be restored when you restart. Model files can also be exported as a .PNG, .JPG, or .SVG file.
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Updated: December 30, 2014