| .M | MathWorks | Arquivos de desenvolvedor | MATLAB Source Code File | gVim | Open Source |
| .M15 | Moving Picture Experts Group | Arquivos de vídeo | MPEG Video File | CocoModX | Open Source |
| .M1A | Open Source | Arquivos de áudio | MPEG-1 Audio File | Media Player Classic | Open Source |
| .M1V | Roxio | Arquivos de vídeo | MPEG-1 Video File | Eltima Elmedia Player | Open Source |
| .M2A | Unknown Developer | Arquivos de vídeo | MPEG-1 Layer 2 Audio File | Media Player Classic | Open Source |
| .M2P | Moving Picture Experts Group | Arquivos de vídeo | MPEG-2 Program Stream File | Media Player Classic | Open Source |
| .M2T | Roxio | Arquivos de vídeo | HDV Video File | Squared 5 MPEG Streamclip | Open Source |
| .M3 | Blizzard Entertainment Inc. | Arquivos 3D | Blizzard MDX3 Model File | m3_Viewer | Open Source |
| .M3U | Nullsoft | Arquivos de áudio | Media Playlist File | Clementine | Open Source |
| .M3U8 | Various Developers | Arquivos de áudio | UTF-8 M3U Playlist File | CocoModX | Open Source |
| .M4 | Open Source | Arquivos de desenvolvedor | Macro Processor Library | AbiWord | Open Source |
| .M4A | Apple | Arquivos de áudio | MPEG-4 Audio File | Eltima Elmedia Player | Open Source |
| .M4S | Ubisoft | Arquivos de jogos | Myst IV: Revelation Save Game File | MP4Box | Open Source |
| .M4V | Apple | Arquivos de vídeo | iTunes Video File | Eltima Elmedia Player | Open Source |
| .M75 | Moving Picture Experts Group | Arquivos de vídeo | MPEG Video File | CocoModX | Open Source |
| .MA2 | Blizzard Entertainment Inc. | Arquivos de jogos | Diablo II Map File | BibExcel | Open Source |
| .MAB | Mozilla | Arquivos miscelâneas | Mozilla Address Book | Markaby | Open Source |
| .MAD | Electronic Arts | Arquivos de jogos | Electronic Arts Madcow Movie File | Media Player Classic | Open Source |
| .MAF | National Cancer Institute | Arquivos de dados | Mutation Annotation Format File | AbiWord | Open Source |
| .MAG | Microsoft Corporation | Arquivos miscelâneas | Access Diagram Shortcut File | Magic VLSI | Open Source |
| .MAL | Open Source | Arquivos de configuração | MadAppLauncher Configuration File | MadAppLauncher | Open Source |
| .MAN | Unknown Developer | Arquivos de Texto | Unix Manual | Pico | Open Source |
| .MAP | id Software | Arquivos de jogos | Quake Engine Map File | Leges Motus | Open Source |
| .MARKDOWN | Various Developers | Arquivos de desenvolvedor | Markdown Documentation File | gVim | Open Source |
| .MBZ | Unknown Developer | Arquivos de complementos | Pogoshell NES Emulator Plugin | Moodle | Open Source |
| .MCA | Mojang | Arquivos de jogos | Minecraft "Anvil" Map File | NBTExplorer | Open Source |
| .MCR | Mojang | Arquivos de jogos | Minecraft Region File | NBTExplorer | Open Source |
| .MD | Various Developers | Arquivos de desenvolvedor | Markdown Documentation File | gVim | Open Source |
| .MD3 | id Software | Arquivos de jogos | Quake 3 3D Model | Cube 2 Sauerbraten | Open Source |
| .MD5 | Ron Rivest | Arquivos miscelâneas | MD5 Checksum File | RapidCRC | Open Source |
| .MDL | MathWorks | Arquivos de dados | MathWorks Simulink Model | ModPlug Player | Open Source |
| .MDR | Olivier Lapicque | Arquivos de áudio | ModPlug Compressed Module | ModPlug Player | Open Source |
| .MDV | Open Source | Arquivos de imagem de disco | QLAY MDV Image | QLAY | Open Source |
| .MDX | Disc Soft Ltd. | Arquivos de imagem de disco | Extended Media Descriptor File | MDXplay | Open Source |
| .ME | Unknown Developer | Arquivos miscelâneas | Temporary Delete Me File | Pico | Open Source |
| .MEA | Malvern Instruments Ltd | Arquivos de dados | Mastersizer 2000 Sample Analysis Data | Meazure | Open Source |
| .MED | Rockwell Automation Inc. | Arquivos de desenvolvedor | RSView Development Project Archive | PSPad | Open Source |
| .MEDLINE | National Library of Medicine | Arquivos de dados | MEDLINE Document | PSPad | Open Source |
| .MEM | Microsoft Corporation | Arquivos de cópia | FoxPro Variable File | GreekToMe | Open Source |
| .MET | Emulex Corporation | Arquivos de dados | eMule Resource File | eMule | Open Source |
| .MFD | Yamaha Corporation | Arquivos de banco de dados | Music Finder Database | ABC Programming langauge | Open Source |
| .MFS | Open Source | Arquivos codificados | MetFS Encrypted File System | MetFS | Open Source |
| .MGA | Open Source | Arquivos de layout de página | Manga Document | Manga Reader | Open Source |
| .MGP | Adobe Systems Incorporated | Arquivos de dados | Adobe Captivate MenuBuilder Data | Magic Point | Open Source |
| .MHA | Open Source | Arquivos Gráficos | MetaImage Medical Format | Insight Segmentation and Registration Toolkit | Open Source |
| .MHP | Open Source | Arquivos de dados | Maths Helper Plus File | Maths Helper Plus | Open Source |
| .MHX | Fujitsu Limited | Arquivos de dados | Fujitsu MicroController Firmware | MakeHuman | Open Source |
| .MI4 | NVIDIA | Arquivos de sistema | Audio Player Firmware File | Rockbox Utility | Open Source |
| .MID | Open Source | Arquivos de áudio | MIDI Audio File | TiMiDity++ | Open Source |
| .MIDI | Synthesia LLC | Arquivos de áudio | MIDI File | Audacity | Open Source |
| .MIF | Adobe Systems Incorporated | Arquivos de layout de página | FrameMaker Interchange Format File | mAirList | Open Source |
| .MIN | Okuma | Arquivos CAD | Okuma CNC Program File | Freemacs | Open Source |
| .MINGW | Open Source | Arquivos de desenvolvedor | MinGW Make File | MinGW | Open Source |
| .MINIPSF | Un4seen Developments | Arquivos de áudio | Miniature PlayStation Sound Format File | XMMS | Open Source |
| .MINT | Open Source | Arquivos comprimidos | Linux Mint Installer File | Linux Mint | Open Source |
| .MIO | Open Source | Arquivos executáveis | MioEngine Application File | MioFactory | Open Source |
| .MKA | Matroska | Arquivos de áudio | Matroska Audio File | Media Player Classic | Open Source |
| .MKD | Malz++Kassner | Arquivos CAD | CAD 6 Drawing File | Daring Fireball Markdown | Open Source |
| .MKR | Microsoft Corporation | Arquivos de jogos | Train Simulator Marker File | my Knowledge Explorer | Open Source |
| .MKS | Matroska | Arquivos de dados | Matroska Elementary Stream File | The Core Media Player | Open Source |
| .MKV | Various Developers | Arquivos de vídeo | Matroska Video File | Eltima Elmedia Player | Open Source |
| .ML | Robin Milner | Arquivos de desenvolvedor | ML Source Code File | gVim | Open Source |
| .MLC | Microsoft Corporation | Arquivos de sistema | Microsoft Language Interface Pack | mAirList | Open Source |
| .MLD | FMJ-Software | Arquivos de áudio | I-Melody Format For I-Mode | mAirList | Open Source |
| .MLP | Meridian Audio | Arquivos de áudio | Meridian Lossless Packing Audio File | mAirList | Open Source |
| .MLS | KIDASA Software, Inc. | Arquivos de dados | Milestones Simplicity File | mAirList | Open Source |
| .MMA | Open Source | Arquivos de dados | Master Album Maker Photo Album File | Master Album Maker Viewer | Open Source |
| .MMP | MixMeister Technology | Arquivos de áudio | MixMeister Studio Playlist | LMMS | Open Source |
| .MMV | Sony | Arquivos de vídeo | MicroMV Video File | Squared 5 MPEG Streamclip | Open Source |
| .MND | Autodesk, Inc. | Arquivos de desenvolvedor | AutoCAD Menu Compiler File | Fractal Forge | Open Source |
| .MNG | Various Developers | Arquivos de imagem Raster | Multiple Network Graphic | IrfanView | Open Source |
| .MO | Dassault Systèmes | Arquivos de dados | Dymola Simulation File | Pidgin | Open Source |
| .MO3 | Un4seen Developments | Arquivos de áudio | MO3 Audio File | XMMS | Open Source |
| .MOBI | Amazon | Arquivos eBook | Mobipocket eBook Format | Calibre | Open Source |
| .MOD | Open Source | Arquivos de áudio | Amiga Music Module File | ModPlug Player | Open Source |
| .MODULE | Open Source | Arquivos de complementos | Drupal Module File | Notepad2 | Open Source |
| .MOFF | Sony | Arquivos de vídeo | Sony Video Data File | GPSBabel | Open Source |
| .MOGG | Unknown Developer | Arquivos de áudio | Multitrack Ogg File | Audacity | Open Source |
| .MOL | Interscape | Arquivos de dados | MDL Molfile | Avogadro | Open Source |
| .MOOD | Open Source | Arquivos de dados | Amarok Moodbar Data File | Amarok | Open Source |
| .MOOV | Apple | Arquivos de vídeo | Apple QuickTime Movie | Eltima Elmedia Player | Open Source |
| .MOT | NewTek | Arquivos 3D | LightWave Motion File | Linux | Open Source |
| .MOV | Apple | Arquivos de vídeo | Apple QuickTime Movie | Eltima Elmedia Player | Open Source |
| .MP | Autodesk, Inc. | Arquivos 3D | Maya PLE Project File | Texmaker | Open Source |
| .MP1 | RealNetworks | Arquivos de áudio | MPEG-1 Layer 1 Audio File | The Core Media Player | Open Source |
| .MP2V | Roxio | Arquivos de vídeo | MPEG-2 Video File | Media Player Classic | Open Source |
| .MP3 | Moving Picture Experts Group | Arquivos de áudio | MP3 Audio File | Eltima Elmedia Player | Open Source |
| .MP4 | Moving Picture Experts Group | Arquivos de vídeo | MPEG-4 Video File | Eltima Elmedia Player | Open Source |
| .MP41 | Microsoft Corporation | Arquivos de vídeo | Microsoft MP41 Video Data | FFDShow | Open Source |
| .MP4V | Moving Picture Experts Group | Arquivos de vídeo | MPEG-4 Video File | Eltima Elmedia Player | Open Source |
| .MPC | Open Source | Arquivos de áudio | Musepack Compressed Audio File | DeliPlayer | Open Source |
| .MPCPL | Open Source | Arquivos miscelâneas | Media Player Classic Playlist | Media Player Classic | Open Source |
| .MPE | Unknown Developer | Arquivos de vídeo | MPEG Movie | Eltima Elmedia Player | Open Source |
| .MPEG | Unknown Developer | Arquivos de vídeo | MPEG Movie | Eltima Elmedia Player | Open Source |
| .MPG | Moving Picture Experts Group | Arquivos de vídeo | MPEG Video File | CocoModX | Open Source |
| .MPG4 | Moving Picture Experts Group | Arquivos de vídeo | MPEG-4 Video File | Eltima Elmedia Player | Open Source |
| .MPGA | RealNetworks | Arquivos de áudio | MPEG-1 Layer 3 Audio File | XMMS | Open Source |
| .MPI | Open Source | Arquivos de dados | InstallJammer Project File | InstallJammer | Open Source |
| .MPK | ESRI | Arquivos GIS | ArcGIS Map Package | Project64 | Open Source |
| .MPKG | Open Source | Arquivos comprimidos | Meta Package File | Linux | Open Source |
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