| .ME | Unknown Developer | Developer Files | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .MEC | Hitachi Industrial Equipment Systems | Developer Files | Ladder Editor Memo Comment | Ladder Editor for Windows | Hitachi Industrial Equipment Systems |
| .MED | Rockwell Automation Inc. | Developer Files | RSView Development Project Archive | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .MEM | Microsoft Corporation | Developer Files | FoxPro Variable File | Microsoft Visual FoxPro | Microsoft Corporation |
| .MER | Rockwell Automation Inc. | Developer Files | RSView Development Runtime File | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .MF | Walt Disney Imagineering | Developer Files | Gameplay Multifile | Panda3D | Walt Disney Imagineering |
| .MFA | Clickteam | Developer Files | Multimedia Fusion Development File | Multimedia Fusion | Interscape |
| .MFCRIBBON-MS | Microsoft Corporation | Developer Files | MFC Ribbon Definition XML File | Microsoft Visual Studio | Microsoft Corporation |
| .MFW | MamiyaLeaf | Developer Files | Mamiya Camera Raw File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .MINGW | Open Source | Developer Files | MinGW Make File | MinGW | Open Source |
| .MIX | Microsoft Corporation | Developer Files | Microsoft Image Exchange File | Microsoft Picture It! | Microsoft Corporation |
| .ML | Robin Milner | Developer Files | ML Source Code File | Notepad++ | Don Ho |
| .MLB | Avanquest Software | Developer Files | MyLabel Designer Deluxe Project File | MyLabel Designer Deluxe | Avanquest Software |
| .MLS | KIDASA Software, Inc. | Developer Files | Milestones Simplicity File | Milestones Professional | KIDASA Software, Inc. |
| .MM | Kodak | Developer Files | Kodak EasyShare Thumbnail Cache File | Kodak EasyShare | Kodak |
| .MMB | Mediachance | Developer Files | MultiMedia Builder Data | MediaChance Multimedia Builder | Mediachance |
| .MND | Autodesk, Inc. | Developer Files | AutoCAD Menu Compiler File | Autodesk AutoCAD | Autodesk, Inc. |
| .MO | Dassault Systèmes | Developer Files | Dymola Simulation File | Dymola | Dassault Systèmes |
| .MOC | Boris FX | Developer Files | MOCHA Java Script | Mocha | Boris FX |
| .MOD | Open Source | Developer Files | Amiga Music Module File | ModPlug Player | Open Source |
| .MODS | Mozilla | Developer Files | Mozilla XULRunner File | Mozilla Firefox | Mozilla |
| .MOM | Apple | Developer Files | Managed Object Model | Apple Xcode | Apple |
| .MPM | Rockstar Games | Developer Files | Max Payne Modification File | Rockstar Games Max Payne | Rockstar Games |
| .MPR | Open Source | Developer Files | Bibexcel Data | BibExcel | Open Source |
| .MQ4 | MetaQuotes Software Corp | Developer Files | MQL4 Source Code File | MetaTrader 4 | MetaQuotes Software Corp |
| .MRL | Sublime HQ Pty Ltd | Developer Files | Multimedia Retrival Markup Language Data | Sublime Text | Sublime HQ Pty Ltd |
| .MRS | MAIET Entertainment | Developer Files | GunZ Game Data File | MAIET Entertainment GunZ | MAIET Entertainment |
| .MRT | Stimulsoft | Developer Files | Stimulsoft Report File | Stimulsoft Reports.Ultimate | Stimulsoft |
| .MSCR | Kaleida Ltd | Developer Files | ServiceMon Script | ServiceMon | Kaleida Ltd |
| .MSH | Martin Schweiger | Developer Files | Orbiter 3D Mesh File | Orbiter | Martin Schweiger |
| .MSH1 | Microsoft Corporation | Developer Files | Microsoft Shell Script File | Microsoft Windows Shell | Microsoft Corporation |
| .MSH2 | Microsoft Corporation | Developer Files | Microsoft Shell Script File | Microsoft Windows Shell | Microsoft Corporation |
| .MSH2XML | Microsoft Corporation | Developer Files | Windows PowerShell Script File | Microsoft Windows PowerShell | Microsoft Corporation |
| .MSHXML | Microsoft Corporation | Developer Files | Windows PowerShell Script File | Microsoft Windows PowerShell | Microsoft Corporation |
| .MSL | Microsoft Corporation | Developer Files | Mapping Specification Language File | Microsoft Notepad | Microsoft Corporation |
| .MSP | Microsoft Corporation | Developer Files | Windows Installer Patch | Windows Installer | Microsoft Corporation |
| .MSS | Xilinx, Inc. | Developer Files | Microprocessor Software Specification File | Xilinx Platform Studio (XPS) | Xilinx, Inc. |
| .MV | Miva | Developer Files | MivaScript File | Miva Merchant Script Compiler | Miva |
| .MVX | Clickteam | Developer Files | Multimedia Fusion Movement Extension | Clickteam Fusion 2.5 | Clickteam |
| .MWP | IBM | Developer Files | Lotus Word Pro SmartMaster File | IBM Lotus Word Pro | IBM |
| .MXML | Adobe Systems Incorporated | Developer Files | Flex MXML Component | Notepad++ | Don Ho |
| .NCB | Microsoft Corporation | Developer Files | Visual C++ IntelliSense Database | Microsoft Visual Studio | Microsoft Corporation |
| .NCF | Valve | Developer Files | No Cache File | GCFScape | Open Source |
| .NCL | National Center for Atmospheric Research (NCAR) | Developer Files | NCAR Command Language File | NCAR Command Language | National Center for Atmospheric Research (NCAR) |
| .NED | OpenSim Ltd | Developer Files | OMNeT++ Network Description File | OMNeT++ | OpenSim Ltd |
| .NFM | Embarcadero Technologies | Developer Files | Delphi .NET Form File | Delphi (IDE) | Embarcadero Technologies |
| .NIB | Apple | Developer Files | Interface Builder User Interface File | Apple Xcode | Apple |
| .NIM | Andreas Rumpf | Developer Files | Nim Source Code File | Nim | Andreas Rumpf |
| .NLS | Uri Wilensky | Developer Files | NetLogo Source Code File | MacroMates TextMate | MacroMates |
| .NNA | NeuralWare | Developer Files | NeuralWare Data | Notepad++ | Don Ho |
| .NNW | NeuralWare | Developer Files | NeuralWorks Data | NeuralWorks | NeuralWare |
| .NQC | Dave | Developer Files | Not Quite C Source Code File | Bricx Command Center | SourceForge |
| .NSE | Gordon Lyon | Developer Files | Nmap security Scanner Scripting Engine File | Nmap Scripting Engine | Gordon Lyon |
| .NSH | Nullsoft | Developer Files | NSIS Header File | NSIS (Nullsoft Scriptable Install System) | SourceForge |
| .NSI | Nullsoft | Developer Files | NSIS Script | NSIS (Nullsoft Scriptable Install System) | SourceForge |
| .NSL | Nullsoft | Developer Files | NsL Script | nsL Assembler for NSIS | Open Source |
| .NUGET | Outercurve Foundation | Developer Files | NuGet Package File | NuGet | Outercurve Foundation |
| .NW | GitHub, Inc. | Developer Files | Node-Webkit App Package | NW.js | GitHub, Inc. |
| .O | Microsoft Corporation | Developer Files | Compiled Object File | Microsoft Visual Studio | Microsoft Corporation |
| .OBE | SourceForge | Developer Files | Objeck Executable File | Objeck | SourceForge |
| .OBR | Micro Focus International plc | Developer Files | Borland C++ Object Browser Data | Borland C++ | Micro Focus International plc |
| .OCA | Microsoft Corporation | Developer Files | Custom Control Library Type File | Microsoft Visual Basic | Microsoft Corporation |
| .OCB | OriginLab | Developer Files | Origin Precompiled Origin C Source Code | Origin | OriginLab |
| .OCTEST | Apple | Developer Files | Xcode Objective-C Unit Test Bundle | Apple Xcode | Apple |
| .OCX | Microsoft Corporation | Developer Files | ActiveX Control | Microsoft Visual Studio | Microsoft Corporation |
| .ODD | Coby Electronics Corporation | Developer Files | Coby Voice Recorder Data | Coby Voice Manager Software | Coby Electronics Corporation |
| .ODL | Microsoft Corporation | Developer Files | Object Description Language File | Microsoft Visual Studio | Microsoft Corporation |
| .OHI | Borland | Developer Files | Delphi OpenHelp Index | Delphi (IDE) | Embarcadero Technologies |
| .OKM | Gian Paolo Saliola | Developer Files | OkMap Map Defining File | OkMap | Gian Paolo Saliola |
| .OLT | Innoshock | Developer Files | Orbit Downloads List | Orbit Downloader | Innoshock |
| .OM | Petter Strandmark | Developer Files | Omicron Compiler | Omicron | Petter Strandmark |
| .OMO | Open Source | Developer Files | OMake Object File | OMake | Open Source |
| .OOM | SAP | Developer Files | PowerDesigner Object-Oriented Model | Sybase PowerDesigner | SAP |
| .OPR | OPRO Japan Co. | Developer Files | OPRO Print Report | OPRO X Browser | OPRO Japan Co. |
| .OPT | LexisNexis | Developer Files | Concordance Database File | LexisNexis Concordance | LexisNexis |
| .OPV | Free Software Foundation | Developer Files | OpenPanorama Visit Data | OpenPanorama Package Manager | Free Software Foundation |
| .ORC | Voyetra Turtle Beach | Developer Files | Voyetra Digital Orchestrator File | Digital Orchestrator | Voyetra Turtle Beach |
| .ORDEREDTEST | Microsoft Corporation | Developer Files | Visual Studio Ordered Test File | Microsoft Visual Studio | Microsoft Corporation |
| .OS | Open Source | Developer Files | ObjectScript Source File | ObjectDevel | Open Source |
| .OSC | OpenStreetMap | Developer Files | OpenStreetMap Change File | Merkaartor | Bart Vanhauwaert, Chris Browet, Ladislav Láska, and others |
| .OW | Open Source | Developer Files | Free Pascal 1.0.x Object File | Free Pascal | Open Source |
| .OWL | Interscape | Developer Files | OWL Source Code File | OWL | Interscape |
| .OX | Open Source | Developer Files | Ox Source Code | OX Professional | Open Source |
| .P | Python Software Foundation | Developer Files | Python Pickle File | Python pickle | Python Software Foundation |
| .P3D | Markus Pfau | Developer Files | Peak3D 3D Graphics File | Peak3D | Markus Pfau |
| .P5 | Larry Wall | Developer Files | Perl Source Code | Microsoft Notepad | Microsoft Corporation |
| .PA3 | Embarcadero Technologies | Developer Files | Turbo Pascal DOS File | Turbo Pascal | Embarcadero Technologies |
| .PAJ | Open Source | Developer Files | Pajek Project File | Pajek | Open Source |
| .PAR | Siemens PLM Software | Developer Files | Solid Edge Part File | Adobe Acrobat DC | Adobe Systems Incorporated |
| .PARTIAL | Microsoft Corporation | Developer Files | Internet Explorer Partially Downloaded File | Internet Explorer | Microsoft Corporation |
| .PAS | Embarcadero Technologies | Developer Files | Delphi Unit Source File | Delphi (IDE) | Embarcadero Technologies |
| .PATCH | Matt Mackall | Developer Files | Mercurial Patch File | Mercurial | Matt Mackall |
| .PAW | SecurelyShare | Developer Files | PawaaFILE File Format | SecurelyShare | SecurelyShare |
| .PB | Fantaisie Software | Developer Files | PureBasic Source File | PureBasic | Fantaisie Software |
| .PBC | Parrot Foundation | Developer Files | Parrot Compiled Byte Code | Parrot | Parrot Foundation |
| .PBI | The FreeBSD Community | Developer Files | PC BSD Installer Package | FreeBSD | The FreeBSD Community |
| .PBL | PowerBASIC, Inc. | Developer Files | PowerBASIC File | PowerBASIC | PowerBASIC, Inc. |
| .PBO | Bohemia Interactive | Developer Files | Packed Bohemia Object | ArmA: Armed Assault | Bohemia Interactive |
| .PBPROJ | Apple | Developer Files | Project Builder Project | Apple Xcode | Apple |
| .PBS | Corel | Developer Files | PaintShop Pro Brush Strokes | PaintShop Pro | Corel |
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