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