| .M | MathWorks | Utviklerfiler | MATLAB Source Code File | Notepad++ | Don Ho |
| .M1S | Newfangled Solutions | Utviklerfiler | Mach3 Macro | Mach3 | Newfangled Solutions |
| .M4 | Open Source | Utviklerfiler | Macro Processor Library | MacroMates TextMate | MacroMates |
| .M4X | INRIA | Utviklerfiler | SynDEx Executive Macro | SynDEx | INRIA |
| .M68 | Embarcadero Technologies | Utviklerfiler | Turbo Pascal DOS File | Turbo Pascal | Embarcadero Technologies |
| .MA3 | Software Publishing Corporation (SPC) | Utviklerfiler | Harvard Graphics Macro | Harvard Graphics Presentation Software | Software Publishing Corporation (SPC) |
| .MAB | Mozilla | Utviklerfiler | Mozilla Address Book | Apache OpenOffice | Apache Software Foundation |
| .MAC | Apple | Utviklerfiler | MacPaint Image | Apple QuickTime Player | Apple |
| .MAK | Sublime HQ Pty Ltd | Utviklerfiler | Makefile | Microsoft Visual Studio | Microsoft Corporation |
| .MAKEFILE | Sublime HQ Pty Ltd | Utviklerfiler | Makefile | Microsoft Visual Studio | Microsoft Corporation |
| .MAP | id Software | Utviklerfiler | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MARKDOWN | Various Developers | Utviklerfiler | Markdown Documentation File | Microsoft Notepad | Microsoft Corporation |
| .MCP | Freescale | Utviklerfiler | CodeWarrior Project | CodeWarrior | NXP |
| .MD | Various Developers | Utviklerfiler | Markdown Documentation File | Microsoft Notepad | Microsoft Corporation |
| .ME | Unknown Developer | Utviklerfiler | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .MEC | Hitachi Industrial Equipment Systems | Utviklerfiler | Ladder Editor Memo Comment | Ladder Editor for Windows | Hitachi Industrial Equipment Systems |
| .MED | Rockwell Automation Inc. | Utviklerfiler | RSView Development Project Archive | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .MEM | Microsoft Corporation | Utviklerfiler | FoxPro Variable File | Microsoft Visual FoxPro | Microsoft Corporation |
| .MER | Rockwell Automation Inc. | Utviklerfiler | RSView Development Runtime File | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .MF | Walt Disney Imagineering | Utviklerfiler | Gameplay Multifile | Panda3D | Walt Disney Imagineering |
| .MFA | Clickteam | Utviklerfiler | Multimedia Fusion Development File | Multimedia Fusion | Interscape |
| .MFCRIBBON-MS | Microsoft Corporation | Utviklerfiler | MFC Ribbon Definition XML File | Microsoft Visual Studio | Microsoft Corporation |
| .MFW | MamiyaLeaf | Utviklerfiler | Mamiya Camera Raw File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .MINGW | Open Source | Utviklerfiler | MinGW Make File | MinGW | Open Source |
| .MIX | Microsoft Corporation | Utviklerfiler | Microsoft Image Exchange File | Microsoft Picture It! | Microsoft Corporation |
| .ML | Robin Milner | Utviklerfiler | ML Source Code File | Notepad++ | Don Ho |
| .MLB | Avanquest Software | Utviklerfiler | MyLabel Designer Deluxe Project File | MyLabel Designer Deluxe | Avanquest Software |
| .MLS | KIDASA Software, Inc. | Utviklerfiler | Milestones Simplicity File | Milestones Professional | KIDASA Software, Inc. |
| .MM | Kodak | Utviklerfiler | Kodak EasyShare Thumbnail Cache File | Kodak EasyShare | Kodak |
| .MMB | Mediachance | Utviklerfiler | MultiMedia Builder Data | MediaChance Multimedia Builder | Mediachance |
| .MND | Autodesk, Inc. | Utviklerfiler | AutoCAD Menu Compiler File | Autodesk AutoCAD | Autodesk, Inc. |
| .MO | Dassault Systèmes | Utviklerfiler | Dymola Simulation File | Dymola | Dassault Systèmes |
| .MOC | Boris FX | Utviklerfiler | MOCHA Java Script | Mocha | Boris FX |
| .MOD | Open Source | Utviklerfiler | Amiga Music Module File | ModPlug Player | Open Source |
| .MODS | Mozilla | Utviklerfiler | Mozilla XULRunner File | Mozilla Firefox | Mozilla |
| .MOM | Apple | Utviklerfiler | Managed Object Model | Apple Xcode | Apple |
| .MPM | Rockstar Games | Utviklerfiler | Max Payne Modification File | Rockstar Games Max Payne | Rockstar Games |
| .MPR | Open Source | Utviklerfiler | Bibexcel Data | BibExcel | Open Source |
| .MQ4 | MetaQuotes Software Corp | Utviklerfiler | MQL4 Source Code File | MetaTrader 4 | MetaQuotes Software Corp |
| .MRL | Sublime HQ Pty Ltd | Utviklerfiler | Multimedia Retrival Markup Language Data | Sublime Text | Sublime HQ Pty Ltd |
| .MRS | MAIET Entertainment | Utviklerfiler | GunZ Game Data File | MAIET Entertainment GunZ | MAIET Entertainment |
| .MRT | Stimulsoft | Utviklerfiler | Stimulsoft Report File | Stimulsoft Reports.Ultimate | Stimulsoft |
| .MSCR | Kaleida Ltd | Utviklerfiler | ServiceMon Script | ServiceMon | Kaleida Ltd |
| .MSH | Martin Schweiger | Utviklerfiler | Orbiter 3D Mesh File | Orbiter | Martin Schweiger |
| .MSH1 | Microsoft Corporation | Utviklerfiler | Microsoft Shell Script File | Microsoft Windows Shell | Microsoft Corporation |
| .MSH2 | Microsoft Corporation | Utviklerfiler | Microsoft Shell Script File | Microsoft Windows Shell | Microsoft Corporation |
| .MSH2XML | Microsoft Corporation | Utviklerfiler | Windows PowerShell Script File | Microsoft Windows PowerShell | Microsoft Corporation |
| .MSHXML | Microsoft Corporation | Utviklerfiler | Windows PowerShell Script File | Microsoft Windows PowerShell | Microsoft Corporation |
| .MSL | Microsoft Corporation | Utviklerfiler | Mapping Specification Language File | Microsoft Notepad | Microsoft Corporation |
| .MSP | Microsoft Corporation | Utviklerfiler | Windows Installer Patch | Windows Installer | Microsoft Corporation |
| .MSS | Xilinx, Inc. | Utviklerfiler | Microprocessor Software Specification File | Xilinx Platform Studio (XPS) | Xilinx, Inc. |
| .MV | Miva | Utviklerfiler | MivaScript File | Miva Merchant Script Compiler | Miva |
| .MVX | Clickteam | Utviklerfiler | Multimedia Fusion Movement Extension | Clickteam Fusion 2.5 | Clickteam |
| .MWP | IBM | Utviklerfiler | Lotus Word Pro SmartMaster File | IBM Lotus Word Pro | IBM |
| .MXML | Adobe Systems Incorporated | Utviklerfiler | Flex MXML Component | Notepad++ | Don Ho |
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