fuzzypeach

http://filext.com/detaillist.php?extdetail=MDI

"A high resolution, tag-based graphics format. .MDI files are only supported by the Microsoft Office 2003 version (and later?) of Microsoft Office Document Imaging. If you are going to share files with people who are using Office Document Imaging 2002, or another document imaging program, save your files in TIFF format for compatibility."