| .M5D | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .M9D | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MAB | Mozilla | Uncommon Files | Mozilla Address Book | Apache OpenOffice | Apache Software Foundation |
| .MAD | Electronic Arts | Uncommon Files | Electronic Arts Madcow Movie File | Media Player Classic | Open Source |
| .MAEGZ | Schrödinger, LLC | Uncommon Files | Compressed Molecular Model File | Maestro | Schrödinger, LLC |
| .MAG | Microsoft Corporation | Uncommon Files | Access Diagram Shortcut File | Microsoft Access | Microsoft Corporation |
| .MANIFEST | Microsoft Corporation | Uncommon Files | Windows Application Manifest File | Microsoft Notepad | Microsoft Corporation |
| .MAP | id Software | Uncommon Files | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MAR | Microsoft Corporation | Uncommon Files | Microsoft Access Report | Microsoft Access | Microsoft Corporation |
| .MAS | Image Space Incorporated | Uncommon Files | rFactor Track File | Image Space rFactor | Image Space Incorporated |
| .MAT | MathWorks | Uncommon Files | MATLAB MAT-File | The MathWorks MATLAB | MathWorks |
| .MAU | Unknown Developer | Uncommon Files | Media Attachment Unit | Unknown Software | Unknown Developer |
| .MB | Autodesk, Inc. | Uncommon Files | Maya Binary Project File | Autodesk Maya | Autodesk, Inc. |
| .MB4 | Evan Designs | Uncommon Files | Model Builder File | Model Builder | Evan Designs |
| .MBB | Kodak | Uncommon Files | Kodak EasyShare Data File | Kodak EasyShare | Kodak |
| .MBFAVS | Pierre-Emmanuel Gougelet | Uncommon Files | Stardent AVS X Bitmap Image | XnView MP | XnSoft |
| .MBG | Fookes Software | Uncommon Files | Mailbag Assistant E-mail Archive File | Mailbag Assistant | Fookes Software |
| .MBS | Opera Software | Uncommon Files | Opera Mailbox File | Opera | Opera Software |
| .MC2 | ValuSoft | Uncommon Files | MasterCook 5 Cookbook File | Mastercook Cookbook | Cosmi Finance LLC |
| .MC6 | Smith Micro Software | Uncommon Files | Poser 6 Material File | Poser | Smith Micro Software |
| .MCF | Jehovah’s Witnesses | Uncommon Files | Watchtower Library Font File | Watchtower Library | Jehovah’s Witnesses |
| .MD0 | Alcohol Soft | Uncommon Files | Alcohol Disk Image Segment 1 | Alcohol 120% | Alcohol Soft |
| .MD5 | Ron Rivest | Uncommon Files | MD5 Checksum File | TeraCopy | Code Sector |
| .MDR | Olivier Lapicque | Uncommon Files | ModPlug Compressed Module | ModPlug Player | Open Source |
| .MDW | Microsoft Corporation | Uncommon Files | Access Workgroup File | Microsoft Access | Microsoft Corporation |
| .ME | Unknown Developer | Uncommon Files | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME2 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MFC | Unknown Developer | Uncommon Files | Mankind File | Unknown Software | Unknown Developer |
| .MHW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MIF | Adobe Systems Incorporated | Uncommon Files | FrameMaker Interchange Format File | Adobe FrameMaker | Adobe Systems Incorporated |
| .MJBOOKTEMPLATE | Mariner Software | Uncommon Files | MacJournal Book Template | Mariner MacJournal | Mariner Software |
| .MJDOC | Mariner Software | Uncommon Files | MacJournal Document | Mariner MacJournal | Mariner Software |
| .MK8 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MKE | Microsoft Corporation | Uncommon Files | Microsoft Windows SDK Make Data | Microsoft .NET Framework | Microsoft Corporation |
| .MKG | GNU Operating System | Uncommon Files | Make Utility File | GNU Make | GNU Operating System |
| .MLI | Unknown Developer | Uncommon Files | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLS | KIDASA Software, Inc. | Uncommon Files | Milestones Simplicity File | Milestones Professional | KIDASA Software, Inc. |
| .MLV | National Instruments | Uncommon Files | LabVIEW Mathscript Data | National Instruments LabVIEW | National Instruments |
| .MMD | Cumulate Labs | Uncommon Files | Cumulate Draw Editable File | Cumulate Draw | Cumulate Labs |
| .MMI | Nortel Networks Corporation | Uncommon Files | Media Processing Server File | Media Processing Server Studio | Nortel Networks Corporation |
| .MMO | Memory-Map | Uncommon Files | Memory Map Overlay File | Memory-Map Navigator | Memory-Map |
| .MMW | Microsoft Corporation | Uncommon Files | Microsoft Clip Organizer Media Catalog | Microsoft Office | Microsoft Corporation |
| .MN4 | Microsoft Corporation | Uncommon Files | Microsoft Money 95 Data File | Microsoft Money | Microsoft Corporation |
| .MN8 | Microsoft Corporation | Uncommon Files | Microsoft Money 2000 Data File | Microsoft Money | Microsoft Corporation |
| .MNL | Autodesk, Inc. | Uncommon Files | AutoCAD Menu LISP File | Autodesk AutoCAD | Autodesk, Inc. |
| .MNW | Unknown Developer | Uncommon Files | Menu File | Unknown Software | Unknown Developer |
| .MNX | Autodesk, Inc. | Uncommon Files | AutoCAD Compiled Menu File | Autodesk AutoCAD | Autodesk, Inc. |
| .MOJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MP4S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MPCPL | Open Source | Uncommon Files | Media Player Classic Playlist | Media Player Classic | Open Source |
| .MQF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MRK | Microsoft Corporation | Uncommon Files | DPOF Auto Print Order File | Microsoft Notepad | Microsoft Corporation |
| .MS-LOCKSCREENCOMPONENT-PRIMARY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MSC | Microsoft Corporation | Uncommon Files | Microsoft C Makefile | Microsoft Visual C++ | Microsoft Corporation |
| .MSEPUB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MSF | Mozilla | Uncommon Files | Mail Summary File | Mozilla Thunderbird | Mozilla |
| .MSI | Microsoft Corporation | Uncommon Files | Windows Installer Package | 7-Zip | Igor Pavlov |
| .MSO | Microsoft Corporation | Uncommon Files | Microsoft Organization Chart Data File | Microsoft Organization Chart | Microsoft Corporation |
| .MSRCINCIDENT | Microsoft Corporation | Uncommon Files | Microsoft Windows Remote Assistance Request Data | Microsoft Windows | Microsoft Corporation |
| .MTA | Samsung Electronics | Uncommon Files | Samsung AllShare Metadata File | Samsung AllShare Play | Samsung Electronics |
| .MTD | Musicnotes | Uncommon Files | Musicnotes Digital Sheet Music File | Musicnotes Player | Musicnotes |
| .MTF | Motorola | Uncommon Files | Motorola Theme File | Motorola MOTOCAST USB | Motorola |
| .MUV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MUZIIC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MVI | Autodesk, Inc. | Uncommon Files | Autodesk AutoCAD Movie Command File | AutoFlix | Autodesk, Inc. |
| .MWF | Autodesk, Inc. | Uncommon Files | MapGuide Author Map Window File | Infrastructure Map Server | Autodesk, Inc. |
| .MWR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MWT | Thomson Reuters Corporation | Uncommon Files | MetaStock Stock Analysis Template | metastock | MetaStock |
| .M_S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
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