| .M | MathWorks | Audio Files | MATLAB Source Code File | Notepad++ | Don Ho |
| .M1A | Open Source | Audio Files | MPEG-1 Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .M2 | Unknown Developer | Audio Files | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M22 | Farallon Computing | Audio Files | Farallon's MacRecorder Recording File | MacRecorder | Farallon Computing |
| .M3A | BirdCage Software | Audio Files | Enhanced M3U Playlist | JukeBox Decoder | BirdCage Software |
| .M3U | Nullsoft | Audio Files | Media Playlist File | Microsoft Windows Media Player | Microsoft Corporation |
| .M3U8 | Various Developers | Audio Files | UTF-8 M3U Playlist File | Microsoft Windows Media Player | Microsoft Corporation |
| .M3V | Unknown Developer | Audio Files | DJ Mix Pro Playlist | DJ Mix Pro | Unknown Developer |
| .M4A | Apple | Audio Files | MPEG-4 Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .M4B | Apple | Audio Files | MPEG-4 Audio Book File | Microsoft Windows Media Player | Microsoft Corporation |
| .M4P | Apple | Audio Files | iTunes Music Store Audio File | Apple iTunes | Apple |
| .M4R | Apple | Audio Files | iPhone Ringtone File | Apple iTunes | Apple |
| .MA1 | Monarch Innovative Technologies Pvt. Ltd | Audio Files | Monarch Audio File | Monarch Digimix | Monarch Innovative Technologies Pvt. Ltd |
| .MAP | id Software | Audio Files | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MBR | GNU Operating System | Audio Files | Master Boot Record File | GNU GRUB | GNU Operating System |
| .MBU | Sony | Audio Files | Sony CAV-M1000ES Memory Backup File | Sony CAV-M1000ES | Sony |
| .MDC | IBM | Audio Files | Multi Dimension Cube File | Cognos software | IBM |
| .MDI | Microsoft Corporation | Audio Files | Microsoft Document Imaging File | Microsoft Office | Microsoft Corporation |
| .MDL | MathWorks | Audio Files | MathWorks Simulink Model | Simulink | MathWorks |
| .MDR | Olivier Lapicque | Audio Files | ModPlug Compressed Module | ModPlug Player | Open Source |
| .MDX | Disc Soft Ltd. | Audio Files | Extended Media Descriptor File | DAEMON Tools | Disc Soft Ltd. |
| .MED | Rockwell Automation Inc. | Audio Files | RSView Development Project Archive | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .MFB | Ensoniq | Audio Files | Ensoniq MR 76 Audio Fata | MR-76 | Ensoniq |
| .MFF | Cakewalk, Inc. | Audio Files | Cakewalk MIDI Format | Cakewalk SONAR | Cakewalk, Inc. |
| .MFK | Keiji Mafuka | Audio Files | Mafuka Audio File | Mafuka Player and Encoder | Keiji Mafuka |
| .MG1 | Microsoft Corporation | Audio Files | MusiGenesis Project | Music Genesis | Microsoft Corporation |
| .MG2 | PG Music Inc | Audio Files | Band-in-a-Box Song File | Band-in-a-Box | PG Music Inc |
| .MGB | Nitro2k01 | Audio Files | Paragon5 Gameboy Tracker Audio | Paragon5 | Nitro2k01 |
| .MGU | PG Music Inc | Audio Files | Band-in-a-Box MIDI Song | Band-in-a-Box | PG Music Inc |
| .MID | Open Source | Audio Files | MIDI Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .MIDI | Synthesia LLC | Audio Files | MIDI File | Microsoft Windows Media Player | Microsoft Corporation |
| .MIF | Adobe Systems Incorporated | Audio Files | FrameMaker Interchange Format File | Adobe FrameMaker | Adobe Systems Incorporated |
| .MINIPSF | Un4seen Developments | Audio Files | Miniature PlayStation Sound Format File | Nullsoft Winamp | Nullsoft |
| .MINIUSF | Nintendo | Audio Files | Nintendo 64 Song File | Nullsoft Winamp | Nullsoft |
| .MIX | Microsoft Corporation | Audio Files | Microsoft Image Exchange File | Microsoft Picture It! | Microsoft Corporation |
| .MIZ | Nullsoft | Audio Files | Winamp Compressed Midi Audio | Nullsoft Winamp | Nullsoft |
| .MJF | Unknown Developer | Audio Files | Mjuice MP3 Audio File | Nullsoft Winamp | Nullsoft |
| .MKA | Matroska | Audio Files | Matroska Audio File | MPlayer | The MPlayer Team |
| .MKF | Interscape | Audio Files | KaraBox Karaoke Music File | KaraBox | Interscape |
| .MKW | Michael K. Weise | Audio Files | Compressed Audio File | mkwACT | Michael K. Weise |
| .MLD | FMJ-Software | Audio Files | I-Melody Format For I-Mode | Awave Studio | FMJ-Software |
| .MLP | Meridian Audio | Audio Files | Meridian Lossless Packing Audio File | MLP Professional Encoder Software | Meridian Audio |
| .MMF | Microsoft Corporation | Audio Files | Microsoft Message File | Microsoft Exchange Server | Microsoft Corporation |
| .MMM | Magix Software GmbH | Audio Files | Music Maker Arrangement File | MAGIX Music Maker | Magix Software GmbH |
| .MMP | MixMeister Technology | Audio Files | MixMeister Studio Playlist | MixMeister Studio | MixMeister Technology |
| .MO3 | Un4seen Developments | Audio Files | MO3 Audio File | XMMS | Open Source |
| .MOD | Open Source | Audio Files | Amiga Music Module File | ModPlug Player | Open Source |
| .MOGG | Unknown Developer | Audio Files | Multitrack Ogg File | Audacity | Open Source |
| .MP1 | RealNetworks | Audio Files | MPEG-1 Layer 1 Audio File | Apple iTunes | Apple |
| .MP2 | New World Computing | Audio Files | Heroes Of Might And Magic II Map File | Heroes of Might & Magic III | Ubisoft |
| .MP3 | Moving Picture Experts Group | Audio Files | MP3 Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .MPA | Unknown Developer | Audio Files | MPEG-2 Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .MPC | Open Source | Audio Files | Musepack Compressed Audio File | VLC Media Player | VideoLAN |
| .MPGA | RealNetworks | Audio Files | MPEG-1 Layer 3 Audio File | Microsoft Windows Media Player | Microsoft Corporation |
| .MPR | Open Source | Audio Files | Bibexcel Data | BibExcel | Open Source |
| .MPY | RCA | Audio Files | RCA Lyra MP3 Player File | MusicMatch Jukebox | Yahoo! |
| .MPZ | Benetech | Audio Files | Miradi Project File | Miradi | Benetech |
| .MP_ | Open Source | Audio Files | Mobile Phone Sound File | BitPim | Open Source |
| .MRA | Unknown Developer | Audio Files | VilagRadio Recorded Radio Program | Unknown Software | Unknown Developer |
| .MSA | Unknown Developer | Audio Files | Magic Shadow Archiver Atari Disk Image | Magic Shadow Archiver | Unknown Developer |
| .MSCZ | Open Source | Audio Files | MuseScore Compressed Score File | MuseScore | Open Source |
| .MSQ | Davidson & Associates, Inc. | Audio Files | Multimedia Workshop Audio File | Multimedia Workshop | Davidson & Associates, Inc. |
| .MSV | Sony | Audio Files | Memory Stick Voice File | Microsoft Windows Media Player | Microsoft Corporation |
| .MT2 | Yannick Delwiche | Audio Files | MadTracker 2 Module | Awave Studio | FMJ-Software |
| .MT9 | Electronics and Telecommunications Research Institute | Audio Files | Music 2.0 Song | Unknown Software | Unknown Developer |
| .MTE | Yannick Delwiche | Audio Files | MadTracker 2 Envelope | MadTracker 2 | Yannick Delwiche |
| .MTF | Motorola | Audio Files | Motorola Theme File | Motorola MOTOCAST USB | Motorola |
| .MTI | Yannick Delwiche | Audio Files | MadTracker Instrument | Awave Studio | FMJ-Software |
| .MTK | Nullsoft | Audio Files | MPU-401 Trakker File | Adlib Tracker II | Adlib Tracker II |
| .MTM | VideoLAN | Audio Files | MultiTracker Module | VLC Media Player | VideoLAN |
| .MTP | Yannick Delwiche | Audio Files | MadTracker 2 Pattern | MadTracker 2 | Yannick Delwiche |
| .MTS | Sony | Audio Files | AVCHD Video File | Microsoft Windows Media Player | Microsoft Corporation |
| .MT_ | Passport Music | Audio Files | Encore Compressed Audio File | Encore | Passport Music |
| .MUI | Myriad Software | Audio Files | Myriad User Instrument File | Myriad Harmony Assistant | Myriad Software |
| .MUK | Unknown Developer | Audio Files | Multak Mini 8162 Encoded Audio Karaoke | Unknown Software | Unknown Developer |
| .MUS | MakeMusic, Inc. | Audio Files | Finale Notation File | SmartMusic | MakeMusic, Inc. |
| .MUSICXML | Michael Good | Audio Files | Compressed MusicXML File | MuseScore | Open Source |
| .MUSX | MakeMusic, Inc. | Audio Files | Finale Notation File | SmartMusic | MakeMusic, Inc. |
| .MUX | Ubisoft | Audio Files | Trackmania Music File | TrackMania | Nadeo |
| .MUZ | HoverDesk.net | Audio Files | MUZ Audio File | Audacity | Open Source |
| .MV3 | Unknown Developer | Audio Files | Audio Data | Unknown Software | Unknown Developer |
| .MW_ | Unknown Developer | Audio Files | Audio Utility Midimach Compressed File | Unknown Software | Unknown Developer |
| .MX3 | Acoustica, Inc. | Audio Files | Mixcraft 3 Audio Project | Acoustica Mixcraft | Acoustica, Inc. |
| .MXL | Michael Good | Audio Files | Compressed MusicXML File | MuseScore | Open Source |
| .MXMF | Nokia | Audio Files | Mobile XMF Ringtone File | Nokia Audio Suite | Nokia |
| .MYR | Myriad Software | Audio Files | Myriad Music File | Myriad Melody Assistant | Myriad Software |
| .MZ | David Webber | Audio Files | Mozart Compressed File | Mozart | David Webber |
| .MZP | Autodesk, Inc. | Audio Files | MAXScript Zip Package | Autodesk 3ds Max | Autodesk, Inc. |
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