| .M2D | HTC | Settings Files | HTC Manila 2D For Windows Mobile Skin | Windows Mobile | Microsoft Corporation |
| .M2F | Maxthon | Settings Files | Maxthon Filter Package Data | Maxthon Cloud Browser | Maxthon |
| .M2S | Maxthon | Settings Files | Maxthon 2 Browser Skin File | Maxthon Cloud Browser | Maxthon |
| .MAILHOST | Microsoft Corporation | Settings Files | MSN Mailhost Settings File | MSN Explorer | Microsoft Corporation |
| .MAL | Open Source | Settings Files | MadAppLauncher Configuration File | MadAppLauncher | Open Source |
| .MASK | Adobe Systems Incorporated | Settings Files | SpeedGrade Color Mask File | Adobe SpeedGrade | Adobe Systems Incorporated |
| .MBL | Vernier Software & Technology | Settings Files | Logger Pro Data File | Vernier Logger Pro | Vernier Software & Technology |
| .MCE | Sony | Settings Files | Sony CAV-M1000ES Common Source Infrared Command File | Sony CAV-M1000ES | Sony |
| .MCH | Interscape | Settings Files | Machinery HDR Effects | Machinery HDR | Interscape |
| .MCL | Microsoft Corporation | Settings Files | Windows Media Center Link File | Microsoft Windows Media Player | Microsoft Corporation |
| .MCV | Visicom Media Inc | Settings Files | ManyCam Effect File | ManyCam | Visicom Media Inc |
| .MDP | Microsoft Corporation | Settings Files | Visual C++ 5 Workspace File | Microsoft Visual C++ | Microsoft Corporation |
| .MDS | SmartBear | Settings Files | TestComplete Project File | TestComplete | SmartBear |
| .MDV | Open Source | Settings Files | QLAY MDV Image | QLAY | Open Source |
| .MD_ | Unknown Developer | Settings Files | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .MEF | Microsoft Corporation | Settings Files | Mamiya RAW Image | ACD Systems ACDSee | ACD Systems |
| .MGK | ImageMagick Studio LLC | Settings Files | ImageMagick Configuration File | ImageMagick | ImageMagick Studio LLC |
| .MHC | Microchip Technology Inc. | Settings Files | MPLAB Harmony Configuration Data | MPLAB Harmony | Microchip Technology Inc. |
| .MIX | Microsoft Corporation | Settings Files | Microsoft Image Exchange File | Microsoft Picture It! | Microsoft Corporation |
| .MKS | Matroska | Settings Files | Matroska Elementary Stream File | The Core Media Player | Open Source |
| .MLB | Avanquest Software | Settings Files | MyLabel Designer Deluxe Project File | MyLabel Designer Deluxe | Avanquest Software |
| .MLC | Microsoft Corporation | Settings Files | Microsoft Language Interface Pack | Microsoft Windows | Microsoft Corporation |
| .MLK | ValuSoft | Settings Files | MasterCook Look File | Mastercook Cookbook | Cosmi Finance LLC |
| .MM4 | Microsoft Corporation | Settings Files | Microsoft Windows Word Processing Format | Microsoft Windows | Microsoft Corporation |
| .MMF | Microsoft Corporation | Settings Files | Microsoft Message File | Microsoft Exchange Server | Microsoft Corporation |
| .MMG | Moritz Bunkus | Settings Files | MKVToolNix Muxing Settings | MKVtoolnix | Interscape |
| .MMH | BioWare | Settings Files | Dragon Age Model Mesh Heirarchy File | Dragon Age Toolset | BioWare |
| .MMP | MixMeister Technology | Settings Files | MixMeister Studio Playlist | MixMeister Studio | MixMeister Technology |
| .MMZ | CREAF and UAB | Settings Files | MiraMon Compressed Map File | MiraMon | CREAF and UAB |
| .MNS | Autodesk, Inc. | Settings Files | AutoCAD Interface Settings File | Autodesk AutoCAD | Autodesk, Inc. |
| .MNU | Autodesk, Inc. | Settings Files | AutoCAD Interface Layout File | Microsoft Notepad | Microsoft Corporation |
| .MO | Dassault Systèmes | Settings Files | Dymola Simulation File | Dymola | Dassault Systèmes |
| .MOF | Microsoft Corporation | Settings Files | Managed Object Format File | Microsoft Systems Management Server | Microsoft Corporation |
| .MPW | Microsoft Corporation | Settings Files | Microsoft Project Workspace File | Microsoft Project 2016 | Microsoft Corporation |
| .MSF | Mozilla | Settings Files | Mail Summary File | Mozilla Thunderbird | Mozilla |
| .MSH1XML | Microsoft Corporation | Settings Files | Monad Display Configuration File | Microsoft Notepad | Microsoft Corporation |
| .MSL | Microsoft Corporation | Settings Files | Mapping Specification Language File | Microsoft Notepad | Microsoft Corporation |
| .MSM | Microsoft Corporation | Settings Files | Windows Installer Merge Module | Windows Installer | Microsoft Corporation |
| .MSN | Microsoft Corporation | Settings Files | Host Blocking File | Microsoft Windows | Microsoft Corporation |
| .MST | Corel | Settings Files | Corel Presentations Master Template File | Corel Presentations X8 | Corel |
| .MSW | Corel | Settings Files | Painter Color Mixer Swatches File | Corel Painter | Corel |
| .MSZ | Ventis Media Inc. | Settings Files | MediaMonkey Skin | MediaMonkey | Ventis Media Inc. |
| .MTF | Motorola | Settings Files | Motorola Theme File | Motorola MOTOCAST USB | Motorola |
| .MTT | Digital Confidence | Settings Files | MetadataTouch Template | Digital Confidence MetadataTouch | Digital Confidence |
| .MU | Squad | Settings Files | Kerbal Space Program Mesh File | Squad Kerbal Space Program | Squad |
| .MUD | Autodesk, Inc. | Settings Files | Mudbox 3D Scene File | Autodesk Mudbox 2017 | Autodesk, Inc. |
| .MXS | Next Limit Technologies | Settings Files | Maxwell Studio Scene File | Maxwell Render | Next Limit Technologies |
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