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.DM2 File Extension

File Type 1	Toad Data Modeler 2 File (Most common)
File Format	File
Common?	No
File Description	Data model created with Toad Data Modeler 2; used for designing databases and developing entity relationship diagrams from existing database files; allows database structures to be represented visually. Charonware is now part of Quest Software. DM2 also refers to a Database Management System (DBMS) developed by IBM.
Program(s) that open dm2 files	Windows: Charonware Toad Data Modeler

File Type 2	Quake 2 Demo File
File Format	Proprietary File
Common?	No
File Description	Recording of a Quake 2 game that can be played back within the Quake 2 program; may be included with the game or recorded by the user; must be saved in the "QUAKE2\BASEQ2\DEMOS" directory in order to be recognized by Quake 2.
Program(s) that open dm2 files	Mac / Windows: id Software Quake 2

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/dm2