| .M2 | Unknown Developer | Spelbestanden | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M3S | Ubisoft | Spelbestanden | Myst III Exile Saved Game | Myst III: Exile | Ubisoft |
| .M4S | Ubisoft | Spelbestanden | Myst IV: Revelation Save Game File | Myst IV: Revelation | Ubisoft |
| .M8 | Raven software | Spelbestanden | Heretic II MipMap Image File | Heretic II | Raven software |
| .M8M | SourceForge | Spelbestanden | 8mam8 Game Data File | 8mam8 | SourceForge |
| .MA | Autodesk, Inc. | Spelbestanden | Maya Project File | Autodesk Maya | Autodesk, Inc. |
| .MA2 | Blizzard Entertainment Inc. | Spelbestanden | Diablo II Map File | Diablo II | Blizzard Entertainment Inc. |
| .MAD | Electronic Arts | Spelbestanden | Electronic Arts Madcow Movie File | Media Player Classic | Open Source |
| .MAE | Interscape | Spelbestanden | My Avatar Editor Character File | My Avatar Editor | Interscape |
| .MAP | id Software | Spelbestanden | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MAPLET | Blitz Research | Spelbestanden | Maplet World File | Blitz Research Maplet | Blitz Research |
| .MAS | Image Space Incorporated | Spelbestanden | rFactor Track File | Image Space rFactor | Image Space Incorporated |
| .MAX | Autodesk, Inc. | Spelbestanden | 3ds Max Scene File | Autodesk 3ds Max | Autodesk, Inc. |
| .MAZ | Microsoft Corporation | Spelbestanden | MAZ | Hover! | Microsoft Corporation |
| .MB1 | Apogee Software, Ltd. | Spelbestanden | Monster Bash Game Data File | Monster Bash | Apogee Software, Ltd. |
| .MCA | Mojang | Spelbestanden | Minecraft "Anvil" Map File | Minecraft | Mojang |
| .MCLEVEL | Mojang | Spelbestanden | Minecraft Map File | Minecraft | Mojang |
| .MCR | Mojang | Spelbestanden | Minecraft Region File | Minecraft | Mojang |
| .MCWORLD | Smith Micro Software | Spelbestanden | Minecraft World Backup File | 7-Zip | Igor Pavlov |
| .MD | Various Developers | Spelbestanden | Markdown Documentation File | Microsoft Notepad | Microsoft Corporation |
| .MD2 | Alcohol Soft | Spelbestanden | Alcohol Disk Image Segment 2 | Alcohol 120% | Alcohol Soft |
| .MD3 | id Software | Spelbestanden | Quake 3 3D Model | QuArK | QuArK Development Team |
| .MDL | MathWorks | Spelbestanden | MathWorks Simulink Model | Simulink | MathWorks |
| .MDM | Scientific Software International, Inc. | Spelbestanden | HLM Multivariate Data Matrix File | SSI HLM software | Scientific Software International, Inc. |
| .MEG | Pennsylvania State University | Spelbestanden | MEGA Data File | MEGA | Pennsylvania State University |
| .MEM | Microsoft Corporation | Spelbestanden | FoxPro Variable File | Microsoft Visual FoxPro | Microsoft Corporation |
| .MENU | id Software | Spelbestanden | Quake 3 Engine Menu File | Microsoft Notepad | Microsoft Corporation |
| .MF | Walt Disney Imagineering | Spelbestanden | Gameplay Multifile | Panda3D | Walt Disney Imagineering |
| .MGL | Microsoft Corporation | Spelbestanden | Age Of Empires 2 Replay File | Microsoft Age of Empires | Ensemble Studios |
| .MGX | Microsoft Corporation | Spelbestanden | Rise Of Nations Saved Game | Rise Of Nations | Big Huge Games |
| .MGZ | KONAMI | Spelbestanden | Metal Gear Solid Archive File | Metal Gear Solid | KONAMI |
| .MII | Nintendo | Spelbestanden | Wii Virtual Avatar File | My Avatar Editor | Interscape |
| .MINE | Mojang | Spelbestanden | Minecraft Map File | Minecraft | Mojang |
| .MIS | GarageGames | Spelbestanden | Marble Blast Gold Mission File | Delta Force | NovaLogic |
| .MIX | Microsoft Corporation | Spelbestanden | Microsoft Image Exchange File | Microsoft Picture It! | Microsoft Corporation |
| .MKR | Microsoft Corporation | Spelbestanden | Train Simulator Marker File | Microsoft Train Simulator | Kuju Entertainment |
| .MLX | MathWorks | Spelbestanden | MATLAB Live Script | The MathWorks MATLAB | MathWorks |
| .MM | Kodak | Spelbestanden | Kodak EasyShare Thumbnail Cache File | Kodak EasyShare | Kodak |
| .MMR | Fuji Xerox | Spelbestanden | Fuji Xerox Fax Image | FaxSee Pro | SFaxTools |
| .MMX | Interscape | Spelbestanden | Command And Conquer Game Data | Command and Conquer | Interscape |
| .MN2 | Interplay Entertainment | Spelbestanden | Descent 2 Mission File | Interplay Descent 2 | Interplay Entertainment |
| .MO | Dassault Systèmes | Spelbestanden | Dymola Simulation File | Dymola | Dassault Systèmes |
| .MOB | Microsoft Corporation | Spelbestanden | MOBTV Video File | Microsoft Windows Media Player | Microsoft Corporation |
| .MOD | Open Source | Spelbestanden | Amiga Music Module File | ModPlug Player | Open Source |
| .MON | Mozilla | Spelbestanden | Firefox Monday Bookmark Backup File | Mozilla Firefox | Mozilla |
| .MOR | Corel | Spelbestanden | Corel Shared Writing Tools Data | Corel Paradox | Corel |
| .MOS | MamiyaLeaf | Spelbestanden | Leaf Camera RAW File | MacPhun ColorStrokes | Macphun |
| .MP2 | New World Computing | Spelbestanden | Heroes Of Might And Magic II Map File | Heroes of Might & Magic III | Ubisoft |
| .MP2M | Rockstar Games | Spelbestanden | Max Payne 2 Mod File | Rockstar Games Max Payne | Rockstar Games |
| .MP2S | Rockstar Games | Spelbestanden | Max Payne 2 Saved Game | Rockstar Games Max Payne | Rockstar Games |
| .MPK | ESRI | Spelbestanden | ArcGIS Map Package | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .MPM | Rockstar Games | Spelbestanden | Max Payne Modification File | Rockstar Games Max Payne | Rockstar Games |
| .MPN | Synergenix Interactive | Spelbestanden | Mophun Game File | Mophun | Synergenix Interactive |
| .MPQ | Blizzard Entertainment Inc. | Spelbestanden | Blizzard Mo'PaQ File | MPQ Editor | Open Source |
| .MPR | Open Source | Spelbestanden | Bibexcel Data | BibExcel | Open Source |
| .MPS | IBM | Spelbestanden | Mathematical Programming System File | Vim | Bram Moolenaar |
| .MPZ | Benetech | Spelbestanden | Miradi Project File | Miradi | Benetech |
| .MQO | tetraface Inc. | Spelbestanden | Metasequoia Document | Metasequoia | tetraface Inc. |
| .MRK | Microsoft Corporation | Spelbestanden | DPOF Auto Print Order File | Microsoft Notepad | Microsoft Corporation |
| .MRS | MAIET Entertainment | Spelbestanden | GunZ Game Data File | MAIET Entertainment GunZ | MAIET Entertainment |
| .MS1 | VirtualBus.info | Spelbestanden | VirtualBus Map File | VirtualBus | Open Source |
| .MSG | Microsoft Corporation | Spelbestanden | Saved Email Message | Microsoft Outlook Express | Microsoft Corporation |
| .MSH | Martin Schweiger | Spelbestanden | Orbiter 3D Mesh File | Orbiter | Martin Schweiger |
| .MSN | Microsoft Corporation | Spelbestanden | Host Blocking File | Microsoft Windows | Microsoft Corporation |
| .MSX | Microsoft Corporation | Spelbestanden | Age Of Empires II Saved Game | Microsoft Age of Empires | Ensemble Studios |
| .MT | Simtex Studios Inc. | Spelbestanden | 1830 Railroads And Robber Barons Game Data | 1830: Railroads & Robber Barons | Simtex Studios Inc. |
| .MTR | Knife Edge Software | Spelbestanden | RealFight Radio Control File | RealFlight | Knife Edge Software |
| .MTV | MyMPX.org | Spelbestanden | MTV Video Format File | ShedWorx Smart Converter | Shedworx |
| .MUL | OriginLab | Spelbestanden | Ultima Online Multi File | MULTool | Interscape |
| .MUS | MakeMusic, Inc. | Spelbestanden | Finale Notation File | SmartMusic | MakeMusic, Inc. |
| .MW0 | Nintendo | Spelbestanden | Super Mario World File | Super Mario World | Nintendo |
| .MW4 | Activision | Spelbestanden | MechWarrior Game Structure File Archive | MechWarrior 2: Mercenaries | Activision |
| .MWM | Keen Software House | Spelbestanden | Space Engineers Model | Space Engineers | Keen Software House |
| .MYS | Vineyard Technologies | Spelbestanden | Vineyard Captured Video File | Vineyard Technologies PC Viewer | Vineyard Technologies |
| .MYT | Burkhard Renk | Spelbestanden | AI Aircraft Traffic Database File | MyTraffic X | Burkhard Renk |
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