| M | Macro module (Brief) |
| M_U | Hard Drive Boot sector backup (MazeGold) |
| M11 | Text file (MASS11) |
| M1U | Pocket TV (PDA) file |
| M1V | MPEG-related file (MIME type 'mpeg') |
| M2P | Canopus MPEG-2 video file |
| M3 | source code file (Modula 3) |
| M3D | 3D animation (Corel Motion) |
| M3U | MPEG URL (MIME audio file) (MP3 Playlist) |
| M4 | M4 Preprocessor file (Unix) |
| M4A | Protected AAC audio file (possibly from iTunes Music Store) |
| M4B | MPEG-4 Audio Layer Bookmarkable File |
| M4P | MPEG-4 Video File |
| M4V | MPEG-4 video file |
| MA3 | Macro (Harvard Graphics 3.0) |
| MAC | Macro (Generic) |
| MAD | Module (Microsoft Access) |
| MAF | Form (Microsoft Access) |
| MAG | MAG graphics format created by Woody Lynn (MPS Magro Paint System) |
| MAGIC | Configuration file (Magic Mail Monitor) |
| MAI | Mail (VAX) |
| MAK | Makefile (Generic) |
| MAM | Macro (Microsoft Access) |
| MAN | Command module (Unix) |
| MAP | Quake Engine game file |
| MAP | Network map (ArcView) |
| MAP | Map file (Generic) |
| MAQ | Query (Microsoft Access) |
| MAR | Assembly program (VAX Macro) |
| MAS | Graphics file (Lotus Freelance Smartmaster) |
| MAT | Table (Microsoft Access) |
| MAUD | Sample format (Maud) |
| MAX | Document (Paperport) |
| MAZ | A format use by Division's dVS/dVISE |
| MB | Memo field values for database (Paradox) |
| MB1 | Data file (Apogee Monster Bash) |
| MBK | Multiple index archive (dBase IV) |
| MBOX | Mailbox file (Berkeley Unix) |
| MBX | Extension Microsoft Outlook adds to saved email, Eudora mailboxes |
| MCC | Configuration file (MathCad) |
| MCD | Document (MathCad) |
| MCF | Magic control file |
| MCI | Command script (Media Control Interface) |
| MCP | Application script (Capsule) |
| MCR | Keyboard macro file (DataCad) |
| MCW | Document (Microsoft Word for Macintosh) |
| MD | Compressed archive (MDCD) |
| MDA | Workgroup (Microsoft Access version 2) |
| MDB | Database (Microsoft Access) |
| MDE | MDE file (Microsoft Access) |
| MDF | Adobe Output Designer and Central Pro Server Compiled Forms Definition |
| MDI | Office Document Imaging file |
| MDL | Music module (MOD) (Digital Trakker) |
| MDM | Modem definition (Telix) |
| MDN | Blank database template (Microsoft Access) |
| MDT | Add-in file (Data) (Microsoft Access) |
| MDW | Workgroup (Microsoft Access) |
| MDX | Multiple index file (dBase IV) |
| MDZ | Wizard template (Microsoft Access) |
| ME | ASCII text document (Generic) |
| MEB | Macro editor bottom overflow library (WordPerfect for Windows) |
| MED | Macro editor delete save file (WordPerfect for Windows) |
| MEM | Memory variable save file (Clipper) |
| MEQ | Macro editor print queue file (WordPerfect for Windows Library) |
| MER | Macro editor resident area (WordPerfect for Windows Library) |
| MES | Message file (Generic) |
| MET | Macro editor top overflow file (WordPerfect for Windows Library) |
| MEU | Menu group (DOS Shell) |
| MEX | Macro editor expound file (WordPerfect for Windows) |
| MF | Metafont text file |
| MFG | Manufacturing file (Pro/ENGINEER) |
| MGF | A file in a Materials and Geometry Format |
| MHP | Microsoft Home Publishing file |
| MHT | MHTML document (Microsoft) |
| MHTM | MHTML document (MIME) |
| MHTML | MHTML document (MIME) |
| MI | Miscellaneous file (Generic) |
| MIC | Image Composer file (Microsoft) |
| MID | MIDI music |
| MIF | Interchange format (Adobe FrameMaker) |
| MIFF | Machine Independent File (Generic) |
| MII | Datafile (MicroStat-II) |
| MIME | See MIM. |
| MIX | Picture file (Microsoft PhotoDraw 2000) |
| MJF | Audio file similar to MP3 (Mjuice) (Opens with WinAmp) |
| MK | Makefile (Generic) |
| MKA | Matroska audio file |
| MKE | Makefile (Microsoft Windows SDK) |
| MKI | Graphic Image (MagView 5.0) (Japanese) |
| MKP | Autodesk Visual Lisp IDE Markup file |
| MKS | Datafile (TACT) |
| MKV | Matroska video file. |
| ML3 | Project file (Milestones 3.x) |
| MLB | Macro library (Symphony) |
| MLI | A file in 3D Studio's Material-Library format |
| MLID | Muliple link interface driver file (Generic) |
| MLM | Groupwise email file (Novell Groupwise) |
| MM | Text file (MultiMate Advantage II) |
| MMAP | MindManager Brainstorm and Process Control Map |
| MMC | Catalog file (Microsoft Clip Gallery 5.x) |
| MMF | Mail file (Microsoft) |
| MMG | Beyond 20/20 table or aggregate data file |
| MML | Bulk mail file (Created by MyMailList) |
| MMM | Multimedia movie (MacroMind Director 3.x) |
| MMO | Memo Writer file (RapidFile) |
| MMP | Output video (Bravado) |
| MN2 | Mission file (Descent2) |
| MN3 | Mission file (Descent3) |
| MND | Mandelbrot for Windows |
| MNG | Multi-image Network Graphics |
| MNI | Mandelbrot for Windows |
| MNT | Menu file (Microsoft FoxPro) |
| MNU | Advanced macro (HP NewWave) |
| MNX | Compiled menu (AutoCAD) |
| MNY | Account book (Microsoft Money) |
| MOB | Device definition (PEN for Windows) |
| MOD | Spreadsheet (Microsoft Multiplan) |
| MOF | Windows Management Object file |
| MON | Monitor description (ReadMail) |
| MOV | Movie (AutoCAD/AutoFlix) |
| MOZ | Zipped (Compressed) mod file |
| MP2 | MPEG Audio Layer 2 file (MIME video file) |
| MP3 | MPEG Audio Layer 3 (AC3) file |
| MP4A | MPEG Audio Layer 4 file |
| MPA | MPEG-related file (MIME type 'mpeg') |
| MPC | Musepack audio file |
| MPD | Database file (Microsoft Project) |
| MPE | MPEG animation |
| MPEG | MPEG animation |
| MPG | MPEG animation |
| MPM | Mathplan Macro library (WordPerfect for Windows) |
| MPP | Drawing file (CAD) |
| MPQ | Blizzard game data file |
| MPR | Menus (compiled) (Microsoft FoxPro) |
| MPT | Microsoft Project template file |
| MPV | View file (Microsoft Project) |
| MPX | Exchange file (Microsoft Project) used for exporting data |
| MPZ | DraCAD (Japanese) file |
| MRB | Multiple resolution bitmap graphics (Microsoft C/C++) |
| MRI | MRI Scan |
| MRS | Macro resource file (WordPerfect for Windows) |
| MSA | Archive (Magic Shadow) |
| MSC | Common console document (Microsoft Windows 2000) |
| MSD | Diagnostics report file (Microsoft MSD) (Diagnostics) |
| MSDL | Scene Description Language (Manchester) |
| MSF | Network 7.0 mail file |
| MSG | Mail message (Microsoft) |
| MSI | Installer package (Microsoft Windows) |
| MSN | Mission File (Descent) |
| MSP | Windows Installer patch file |
| MSP | Paint bitmap (Microsoft) |
| MSS | Manuscript text file (Scribble) |
| MST | Minispecification file (Prosa) |
| MSV | Sony Memory Stick Compressed Voice File; requires Sony plug-in for Windows Media Player |
| MSW | Text file (Microsoft Word) |
| MSX | Compressed CP/M archive (MSX) |
| MTH | Math file (Derive) |
| MTM | Music module (MOD) (Multitracker) |
| MTS | AVCHD video clip format |
| MTW | Datafile (Minitab) |
| MTX | Twain device driver (32 bit)(UMax) |
| MU | Menu (Quattro Pro) |
| MUL | Online game called Ultima online |
| MUS | Interactive music audio data file (Electronic Arts) |
| MUS10 | Audio (Mus10) |
| MV | Server-side script file (Miva) |
| MVA | Video accelerator file (Matrox) |
| MVB | Multimedia Viewer file (Microsoft) |
| MVC | Image file (Sony Digital Mavica) |
| MVE |
Interplay video file (Descent2, Fallout2) |
| MVF |
Stop frame file (AutoCAD-AutoFlix) |
| MVI |
Movie command file (AutoCAD-AutoFlix) |
| MVL |
Descent 2 (game) movie archive |
| MVL |
3D ultrasound file |
| MVL |
Interplay multimedia file |
| MVW |
Log file (Saber LAN) |
| MWF |
Animation (ProMotion) |
| MWP |
Smartmaster file (Lotus WordPro 97) |
| MXD |
Map file (ArcInfo) |
| MXT |
Datafile (Microsoft C) |
| MYP |
Presentation file (Make Your Point) |
