There is only one file type that uses the .MDI extension:
Microsoft Document Imaging File
An MDI file is a document saved in a high-resolution, tag-based graphics format developed from the TIFF format. It is created by Microsoft Office to save scanned documents and may be turned into a Word (.DOC) document using OCR (optical character recognition) software.
MDI files are created and opened with Microsoft Office Document Imaging (MODI), which is included with Office XP, 2003, and 2007. Microsoft stopped including MODI with Office 2010, so you cannot open MDI files in Office 2010. Saving documents created with MODI in the .TIF format allows for better compatibility with other programs.
Microsoft provides the "MDI to TIFF File Converter," which is a command-line tool that can be used to convert MDI files to .TIFF files. You can also use MDI Converter or Bugysoft MDI2DOC to convert MDI files.
Updated: April 3, 2020