There is only one file type that uses the .MDR extension:
ModPlug Compressed Module
Compressed music module file created by ModPlug Player, an application used to play module music files; stores a module such as an .MTM, .MT2, or .IT file that is compressed with RAR compression.
ModPlug Player's ability to compress supported modules enables more convenient sharing between users. The application provides a variety of compression formats, which include Zip, LHA, and RAR. ModPlug Player is the only application that supports the MDR format.
NOTE: ModPlug Player was formerly known as ModPlug Tracker.
Updated: May 3, 2015