| .MJBOOKTEMPLATE | Mariner Software | Ikke Almindelige Filer | MacJournal Book Template | Mariner MacJournal | Mariner Software |
| .MJDOC | Mariner Software | Ikke Almindelige Filer | MacJournal Document | Mariner MacJournal | Mariner Software |
| .MK8 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MKE | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Windows SDK Make Data | Microsoft .NET Framework | Microsoft Corporation |
| .MKG | GNU Operating System | Ikke Almindelige Filer | Make Utility File | GNU Make | GNU Operating System |
| .MLI | Unknown Developer | Ikke Almindelige Filer | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLS | KIDASA Software, Inc. | Ikke Almindelige Filer | Milestones Simplicity File | Milestones Professional | KIDASA Software, Inc. |
| .MLV | National Instruments | Ikke Almindelige Filer | LabVIEW Mathscript Data | National Instruments LabVIEW | National Instruments |
| .MMD | Cumulate Labs | Ikke Almindelige Filer | Cumulate Draw Editable File | Cumulate Draw | Cumulate Labs |
| .MMI | Nortel Networks Corporation | Ikke Almindelige Filer | Media Processing Server File | Media Processing Server Studio | Nortel Networks Corporation |
| .MMO | Memory-Map | Ikke Almindelige Filer | Memory Map Overlay File | Memory-Map Navigator | Memory-Map |
| .MMW | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Clip Organizer Media Catalog | Microsoft Office | Microsoft Corporation |
| .MN4 | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Money 95 Data File | Microsoft Money | Microsoft Corporation |
| .MN8 | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Money 2000 Data File | Microsoft Money | Microsoft Corporation |
| .MNL | Autodesk, Inc. | Ikke Almindelige Filer | AutoCAD Menu LISP File | Autodesk AutoCAD | Autodesk, Inc. |
| .MNW | Unknown Developer | Ikke Almindelige Filer | Menu File | Unknown Software | Unknown Developer |
| .MNX | Autodesk, Inc. | Ikke Almindelige Filer | AutoCAD Compiled Menu File | Autodesk AutoCAD | Autodesk, Inc. |
| .MOJ | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MP4S | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MPCPL | Open Source | Ikke Almindelige Filer | Media Player Classic Playlist | Media Player Classic | Open Source |
| .MQF | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MRK | Microsoft Corporation | Ikke Almindelige Filer | DPOF Auto Print Order File | Microsoft Notepad | Microsoft Corporation |
| .MS-LOCKSCREENCOMPONENT-PRIMARY | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MSC | Microsoft Corporation | Ikke Almindelige Filer | Microsoft C Makefile | Microsoft Visual C++ | Microsoft Corporation |
| .MSEPUB | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MSF | Mozilla | Ikke Almindelige Filer | Mail Summary File | Mozilla Thunderbird | Mozilla |
| .MSI | Microsoft Corporation | Ikke Almindelige Filer | Windows Installer Package | 7-Zip | Igor Pavlov |
| .MSO | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Organization Chart Data File | Microsoft Organization Chart | Microsoft Corporation |
| .MSRCINCIDENT | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Windows Remote Assistance Request Data | Microsoft Windows | Microsoft Corporation |
| .MTA | Samsung Electronics | Ikke Almindelige Filer | Samsung AllShare Metadata File | Samsung AllShare Play | Samsung Electronics |
| .MTD | Musicnotes | Ikke Almindelige Filer | Musicnotes Digital Sheet Music File | Musicnotes Player | Musicnotes |
| .MTF | Motorola | Ikke Almindelige Filer | Motorola Theme File | Motorola MOTOCAST USB | Motorola |
| .MUV | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MUZIIC | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MVI | Autodesk, Inc. | Ikke Almindelige Filer | Autodesk AutoCAD Movie Command File | AutoFlix | Autodesk, Inc. |
| .MWF | Autodesk, Inc. | Ikke Almindelige Filer | MapGuide Author Map Window File | Infrastructure Map Server | Autodesk, Inc. |
| .MWR | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .MWT | Thomson Reuters Corporation | Ikke Almindelige Filer | MetaStock Stock Analysis Template | metastock | MetaStock |
| .M_S | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .N19 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NAB | NetScape | Ikke Almindelige Filer | Netscape Communicator Addressbook | Netscape Navigator | NetScape |
| .NAV | Valve | Ikke Almindelige Filer | Id Tech 3 Bot Navigation File | Quake 3 Arena | id Software |
| .NCP2 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NDJ | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NDL | IBM | Ikke Almindelige Filer | Lotus Notes Doclink File | IBM Lotus Notes | IBM |
| .NET0143338 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NFG | Open Source | Ikke Almindelige Filer | NewsBin Configuration File | DJI Interprises NewsBin Pro | Open Source |
| .NGF | Enterasys Networks | Ikke Almindelige Filer | Enterasys Networks NetSight Generated Format File | NetSight | Enterasys Networks |
| .NGM | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NICK | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Outlook Nickname File | Microsoft Outlook | Microsoft Corporation |
| .NJB | Nikon | Ikke Almindelige Filer | Nikon Photo Index File | Nikon Capture NX | Nikon |
| .NJP | Unknown Developer | Ikke Almindelige Filer | NjPipes Compiler File | Unknown Software | Unknown Developer |
| .NK2 | Microsoft Corporation | Ikke Almindelige Filer | Outlook 2002 Nickname File | Microsoft Outlook | Microsoft Corporation |
| .NLX | PowerUp Software | Ikke Almindelige Filer | FormWorx Form Document File | FormWorx | PowerUp Software |
| .NNB | Oolution Technologies | Ikke Almindelige Filer | ANNI Batch File | ANNI SOFTWARE | Oolution Technologies |
| .NNX | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NOC | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NODE1 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NOMEDIA | Google | Ikke Almindelige Filer | Android No Media File | StudioKUMA .nomedia Manager | Studio KUMA |
| .NPC | Nikon | Ikke Almindelige Filer | Picture Control Utility Picture Control File | Picture Control Utility | Nikon |
| .NQI | ESET | Ikke Almindelige Filer | NOD32 Quarantine File | ESET Nod32 Antivirus | ESET |
| .NSS | Perion Network Ltd | Ikke Almindelige Filer | Magentic Screensaver File | IncrediMail | Perion Network Ltd |
| .NSU | NVIDIA | Ikke Almindelige Filer | NVIDIA nTune Command System Profile Settings | NVIDIA nTune | NVIDIA |
| .NTF | Open Source | Ikke Almindelige Filer | Windows Font File | Microsoft Windows | Microsoft Corporation |
| .NTH | Nokia | Ikke Almindelige Filer | Nokia Series 40 Theme File | Nokia Series 40 Theme Studio | Nokia |
| .NT_ | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .NUL | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NUP | ESET | Ikke Almindelige Filer | Smart Security Update File | ESET Nod32 Antivirus | ESET |
| .NVI | NVIDIA | Ikke Almindelige Filer | NVIDIA Driver File | NVIDIA Graphics Driver | NVIDIA |
| .NWG | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NWM | Sony | Ikke Almindelige Filer | Sony NWM Display Screen File | Adobe Photoshop | Adobe Systems Incorporated |
| .NWP | Perion Network Ltd | Ikke Almindelige Filer | Magentic Wallpaper File | IncrediMail | Perion Network Ltd |
| .NXA | Fipertec S.à.r.l. | Ikke Almindelige Filer | NanoTrader Nano Export Archive File | NanoTrader | Fipertec S.à.r.l. |
| .NXV | Silicon Graphics, Inc. | Ikke Almindelige Filer | NXV Video Data | Video Format Combiner | Silicon Graphics, Inc. |
| .OAS | Fujitsu Limited | Ikke Almindelige Filer | Fujitsu OAS Document File | OASYS Viewer | Fujitsu Limited |
| .OB | Innoshock | Ikke Almindelige Filer | Orbit Downloader Incomplete Download | Orbit Downloader | Innoshock |
| .OB! | Innoshock | Ikke Almindelige Filer | Orbit Downloader Incomplete Download | Orbit Downloader | Innoshock |
| .OCR | URUWORKS | Ikke Almindelige Filer | Subtitle Workshop OCR Script | URUWorks Subtitle Workshop | URUWORKS |
| .ODCCUBEFILE | Apache Software Foundation | Ikke Almindelige Filer | OpenDocument 1 Chart | Apache OpenOffice | Apache Software Foundation |
| .ODF | Oracle Corporation | Ikke Almindelige Filer | OpenDocument Formula | Apache OpenOffice | Apache Software Foundation |
| .ODK | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .ODTX | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .OED | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OF | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .OFL_EN | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .OLB | Microsoft Corporation | Ikke Almindelige Filer | OLE Object Library | Microsoft Excel | Microsoft Corporation |
| .ON | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .ONION | Unknown Developer | Ikke Almindelige Filer | Dharma Ransomware Affected Data | Unknown Software | Unknown Developer |
| .ONR | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OO | Open Source | Ikke Almindelige Filer | Free Pascal Object File For OS/2 Version | Free Pascal | Open Source |
| .OOK | Michal Polak | Ikke Almindelige Filer | Arachne Batch Script | Arachne | Michal Polak |
| .OPF | E-Book Systems | Ikke Almindelige Filer | FlipViewer FlipBook File | FlipViewer | E-Book Systems |
| .ORCA | Unknown Developer | Ikke Almindelige Filer | Unknown Apple II Data File | Unknown Software | Unknown Developer |
| .OSA | Symantec Corporation | Ikke Almindelige Filer | W32/Sober-AD File | AVG Antivirus | AVG Technologies |
| .OST | Microsoft Corporation | Ikke Almindelige Filer | Outlook Offline Data File | Microsoft Outlook 2016 | Microsoft Corporation |
| .OTC | Apache Software Foundation | Ikke Almindelige Filer | OpenDocument Chart Template | Apache OpenOffice | Apache Software Foundation |
| .OUB | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OUF | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .OUI | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .OUR | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
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