| .IHT | Unknown Developer | Developer Files | Intranet Connector Script File | Unknown Software | Unknown Developer |
| .II | Axialis Company | Developer Files | IconWorkshop Extended Information File | Axialis IconWorkshop | Axialis Company |
| .IJS | Open Source | Developer Files | J Interpretor Script | J language | Open Source |
| .IJX | Jsoftware | Developer Files | J Language Execution Window | J language | Open Source |
| .IK | Ola Bini | Developer Files | Ioke Script | Ioke | Ola Bini |
| .ILE | Cadence Design Systems | Developer Files | Encrypted Cadence SKILL File | SKILL | Cadence Design Systems |
| .ILK | Microsoft Corporation | Developer Files | Microsoft Visual Studio Linker Input File | Microsoft Visual Studio | Microsoft Corporation |
| .IMI | Embarcadero Technologies | Developer Files | Turbo Pascal DOS File | Turbo Pascal | Embarcadero Technologies |
| .IML | JetBrains | Developer Files | IntelliJ IDEA Module | Microsoft Notepad | Microsoft Corporation |
| .IMP | Lotus Software | Developer Files | Improv Spreadsheet File | Lotus 1-2-3 | IBM |
| .INC | Microsoft Corporation | Developer Files | Include File | MacroMates TextMate | MacroMates |
| .INH | Microsoft Corporation | Developer Files | Visual Basic Basfwin File | Microsoft Visual Basic | Microsoft Corporation |
| .INL | Microsoft Corporation | Developer Files | C++ Inline File | C++ | Bjarne Stroustrup |
| .INO | Jordan Russell | Developer Files | Inno Setup Script | Microsoft Notepad | Microsoft Corporation |
| .IPP | The Eclipse Foundation | Developer Files | Inline Guard Macro File | Microsoft Visual Studio | Microsoft Corporation |
| .IPR | Flexera Software | Developer Files | InstallShield Project | InstallShield | Flexera Software |
| .IPS | ZeroSoft Zophar | Developer Files | Internal Patching System Patch File | MultiPatch patching tool | Open Source |
| .IQT | Tektronix | Developer Files | Tektronix File | The MathWorks MATLAB | MathWorks |
| .IS | elego Software Solutions GmbH | Developer Files | Intermediate Assembly File | Modula-3 | elego Software Solutions GmbH |
| .ISA | Adobe Systems Incorporated | Developer Files | Adobe Photoshop ImageReady Action Data | Adobe Photoshop | Adobe Systems Incorporated |
| .ISC | Xilinx, Inc. | Developer Files | Xilinx Device Configuration File | Xilinx Platform Studio (XPS) | Xilinx, Inc. |
| .ISM | Microsoft Corporation | Developer Files | IIS Smooth Streaming Manifest File | Microsoft IIS | Microsoft Corporation |
| .IST | Flexera Software | Developer Files | InstallShield Project Template File | InstallShield | Flexera Software |
| .IVU | IMMERVISION ENABLES | Developer Files | ImmerVision XML User Interface | PURE Player | IMMERVISION ENABLES |
| .IWB | Ionic Wind | Developer Files | IWBasic Source Code File | IonicWind IWBasic | Ionic Wind |
| .IWS | JetBrains | Developer Files | IntelliJ IDEA Web Page | Internet Explorer | Microsoft Corporation |
| .IWZ | Flexera Software | Developer Files | InstallShield Express 2 Project File | InstallShield | Flexera Software |
| .IX3 | Lokasoft | Developer Files | Rebel Chess Program File | Rebel | Lokasoft |
| .IXX | Bjarne Stroustrup | Developer Files | C++ Include File | C++ | Bjarne Stroustrup |
| .IZS | Evrsoft | Developer Files | 1st Page 2000 JavaScript Example File | 1st Page 2000 | Evrsoft |
| .J | Oracle Corporation | Developer Files | Java Source File | Vim | Bram Moolenaar |
| .JAV | Oracle Corporation | Developer Files | Java Source Code | Microsoft Notepad | Microsoft Corporation |
| .JAVA | Oracle Corporation | Developer Files | Java Source Code | Microsoft Notepad | Microsoft Corporation |
| .JCP | Xinox Software | Developer Files | JCreator Project File | JCreator | Xinox Software |
| .JCS | Oracle Corporation | Developer Files | Java Control Source Data | Java | Oracle Corporation |
| .JED | Xilinx, Inc. | Developer Files | Xilinix JEDEC Programming File | iMPACT | Xilinx, Inc. |
| .JL | Julian Gabriel Casadesus and Others | Developer Files | Julia Source Code File | Julia | Open Source |
| .JLC | GNOME | Developer Files | Byte Compiled Lisp-Based Script File | Sawfish | GNOME |
| .JPR | Microsoft Corporation | Developer Files | WordPad Document | Microsoft WordPad | Microsoft Corporation |
| .JPX | Embarcadero Technologies | Developer Files | JBuilder Project | JBuilder | Embarcadero Technologies |
| .JS | NetScape | Developer Files | JavaScript File | Notepad++ | Don Ho |
| .JSB | Freedom Scientific | Developer Files | JAWS Script Binary File | JAWS | Freedom Scientific |
| .JSFL | Adobe Systems Incorporated | Developer Files | Animate JSFL Script File | Adobe Animate CC 2017 | Adobe Systems Incorporated |
| .JSH | Freedom Scientific | Developer Files | JAWS Script Header File | JAWS | Freedom Scientific |
| .JSM | Mozilla | Developer Files | Firefox JavaScript Module | Mozilla Firefox | Mozilla |
| .JSPF | Oracle Corporation | Developer Files | Java Server Page Fragment | Adobe Dreamweaver | Adobe Systems Incorporated |
| .JSS | SourceForge | Developer Files | JavaScript Style Sheet File | JSS (Javascript Style Sheets) | SourceForge |
| .JTK | Sun Microsystems | Developer Files | Java ToolKit File | Java | Oracle Corporation |
| .KB | Borland | Developer Files | C++ Keyboard Script | knowledge pro software | Knowledge Garden Inc |
| .KDEVELOP | Free Software Foundation | Developer Files | KDevelop Project Data File | KDevelop | Open Source |
| .KDEVPRJ | Free Software Foundation | Developer Files | KDevelop Project File | KDevelop | Open Source |
| .KED | Mansfield Software Group | Developer Files | KEDIT Profile And Macro File | Kedit | Mansfield Software Group |
| .KL | Haxial Software | Developer Files | Haxial KL Source Code File | Haxial Compiler | Haxial Software |
| .KM3 | The Eclipse Foundation | Developer Files | Kernel MetaMetaModel File | Eclipse IDE for Java Developers | The Eclipse Foundation |
| .KPL | Phrogsoft | Developer Files | Kids Programming Language Source Code | Phrogram | Phrogsoft |
| .KPS | Tavultesoft | Developer Files | Keyman Developer Package Source File | Keyman Developer | Tavultesoft |
| .KST | Olivetti | Developer Files | Olivetti Olitext Plus Script | Olitext Plus | Olivetti |
| .KTT | MJMSoft Design | Developer Files | KeyText Data File | KeyText | MJMSoft Design |
| .KVS | Open Source | Developer Files | KVIrc Script | KVIrc | Open Source |
| .L | Johan Bontes | Developer Files | Game Of Life File | Life32 | Johan Bontes |
| .LBI | Adobe Systems Incorporated | Developer Files | Dreamweaver Library Item | Adobe Dreamweaver | Adobe Systems Incorporated |
| .LBS | Omnis Software | Developer Files | Omnis Library | Omnis Studio | Omnis Software |
| .LBXUPD | Interscape | Developer Files | Libronix DLS Update Script | Logos Bible Software | Interscape |
| .LDS | GNU Project | Developer Files | Binutils LD Linker Script | Microsoft Notepad | Microsoft Corporation |
| .LHS | SourceForge | Developer Files | Literate Haskell Script | Microsoft Notepad | Microsoft Corporation |
| .LIS | SourceForge | Developer Files | SQR (Structured Query Reporting) Output File | Microsoft Notepad | Microsoft Corporation |
| .LIST | Open Source | Developer Files | APT List File | Advanced Package Tool (APT) | Open Source |
| .LIT | Microsoft Corporation | Developer Files | Microsoft Reader eBook File | Calibre | Open Source |
| .LLP | General Electric Company | Developer Files | VersaPro Local Logic Block | VersaPro | General Electric Company |
| .LMP | id Software | Developer Files | Quake Engine Lump File | Doom | id Software |
| .LNP | Keil | Developer Files | µVision Linker Input Data | µVision4 | Keil |
| .LNT | Gimpel Software | Developer Files | PC-lint/FlexeLint Configuration File | Microsoft Notepad | Microsoft Corporation |
| .LNX | Commodore International | Developer Files | Commodore 64 Lynx Archive | VICE | Open Source |
| .LOCAL | Embarcadero Technologies | Developer Files | Delphi User-Specific Project Options | Delphi (IDE) | Embarcadero Technologies |
| .LOL | Apple | Developer Files | LOLCODE Source Code FIle | LibreOffice | The Document Foundation |
| .LPF | Orgadata AG | Developer Files | Orgadata LogiKal File | LogiKal | Orgadata AG |
| .LPROJ | Apple | Developer Files | Localized Project Folder | macOS | Apple |
| .LRF | Aequus Gaming LTD | Developer Files | League Of Legends Replay File | LOLReplay | Aequus Gaming LTD |
| .LSP | Open Source | Developer Files | Lisp Program Source Code File | MacroMates TextMate | MacroMates |
| .LSS | IBM | Developer Files | LotusScript Source Code | IBM Domino | IBM |
| .LTB | LithTech | Developer Files | LithTech Binary File | Lith Tech Jupiter | LithTech |
| .LUA | Unknown Developer | Developer Files | Lua Source File | Microsoft Notepad | Microsoft Corporation |
| .LUAC | L. H. de Figueiredo, R. Ierusalimschy and W. Celes | Developer Files | Lua Compiled Source Code | LUAC | L. H. de Figueiredo, R. Ierusalimschy and W. Celes |
| .LUC | SourceForge | Developer Files | Compiled Lua Source File | LUA | Interscape |
| .LUCA | Unknown Developer | Developer Files | LUCA Templates | Java | Oracle Corporation |
| .LUCIDSNIPPET | The Escapers | Developer Files | Lucid Snippet File | Lucid | The Escapers |
| .M | MathWorks | Developer Files | MATLAB Source Code File | Notepad++ | Don Ho |
| .M1S | Newfangled Solutions | Developer Files | Mach3 Macro | Mach3 | Newfangled Solutions |
| .M4 | Open Source | Developer Files | Macro Processor Library | MacroMates TextMate | MacroMates |
| .M4X | INRIA | Developer Files | SynDEx Executive Macro | SynDEx | INRIA |
| .M68 | Embarcadero Technologies | Developer Files | Turbo Pascal DOS File | Turbo Pascal | Embarcadero Technologies |
| .MA3 | Software Publishing Corporation (SPC) | Developer Files | Harvard Graphics Macro | Harvard Graphics Presentation Software | Software Publishing Corporation (SPC) |
| .MAB | Mozilla | Developer Files | Mozilla Address Book | Apache OpenOffice | Apache Software Foundation |
| .MAC | Apple | Developer Files | MacPaint Image | Apple QuickTime Player | Apple |
| .MAK | Sublime HQ Pty Ltd | Developer Files | Makefile | Microsoft Visual Studio | Microsoft Corporation |
| .MAKEFILE | Sublime HQ Pty Ltd | Developer Files | Makefile | Microsoft Visual Studio | Microsoft Corporation |
| .MAP | id Software | Developer Files | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MARKDOWN | Various Developers | Developer Files | Markdown Documentation File | Microsoft Notepad | Microsoft Corporation |
| .MCP | Freescale | Developer Files | CodeWarrior Project | CodeWarrior | NXP |
| .MD | Various Developers | Developer Files | Markdown Documentation File | Microsoft Notepad | Microsoft Corporation |
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