| j3o | JMonkey Engine 3D Object |
| jacl | IBM WebSphere Server ACL Script |
| jacl | Java Application Control Language Script |
| jadd | JastAdd Imperative Aspect |
| jade | Jade Markup Language Document |
| jak | AHEAD Tool Suite JAK File |
| jal | JAL Source Code |
| jam | Jam STAPL Source Code |
| jardesc | Eclipse JAR Description |
| jasper | JasperReports Compiled JRXML File |
| java | Java Source Code |
| javajet | Eclipse JET Java Template |
| jba | Jabaco Source Code |
| jbc | Jam STAPL Byte-Code |
| jcc | JX C++ Source Code |
| jci | PSDsoft Express JTAG Chain Information |
| jck | Microsoft Liquid Motion JACK Scene Description |
| jcl | Job Control Language File |
| jcm | JECF Java Commerce Message |
| jco | CelLab Own Code |
| jcs | WebLogic Java Control Source |
| jcx | Java Control Extensions File |
| jdbc | JDBC Configuration |
| jdi | Altera Quartus II JTAG Debugging Information File |
| jfd | JFormDesigner Form Data |
| jfg | JForge Binary Code |
| jfr | Java Flight Recorder Data |
| jfr | JRockit Flight Recording File |
| jfs | JRockit Flight Recorder Template |
| jfsl | JForge Specification Language Document |
| jh | JHipster Source Code |
| jhtm | Dynamo Application Server Page |
| ji | Join-Calculus Interface Source Code |
| jic | Altera Quartus II JTAG Indirect Configuration |
| jil | Java in Lisp Source Code |
| jin | Java Servlet Include |
| jio | Join-Calculus JCC Compiled Interface |
| jj | JavaCC Data |
| jjs | Nashorn JJS Script |
| jl | Julia Source Code |
| jl | Sawfish Lisp-based Script Source Code |
| jld | Julia HDF5 Data |
| jlr | Juliar Source Code |
| jlrlib | Juliar Library |
| jlx | JLex Code |
| jmap | CodeWarrior JMAP Data |
| jmdac | Modelio Compiled Module |
| jmf | Java Media Framework Data |
| jmk | JX Makefile |
| jml | Java Modeling Language Specification Data |
| jmo | JMonkey Engine Model |
| jnilib | Java Native Interface Library |
| jomproj | Visual Studio JOOB Object Model Project |
| jop | JOP JOPizer Generated Bytecode |
| jor | Gupta SQLWindows Journal Data |
| joy | CryENGINE Facial Editor Joystick Settings |
| jpage | Eclipse IDE Java Scrapbook Page |
| jpb | J Publish Manager Data |
| jpc | JProbe Coverage Memory Snapshot |
| jpd | Java Process Definition |
| jpf | Java PathFinder Application Property Data |
| jph | JProbe Memory Snapshot |
| jpl | JProbe Settings |
| jpq | JPQ Library |
| jpt | JProbe Threadalyzer Memory Snapshot |
| jpy | JPY Data |
| jr | JR Source Code |
| jrag | JastAdd Declarative Aspect |
| jrprint | JasperReports Print Document |
| jrxml | JasperReports Document |
| js | JavaScript |
| js1 | Unity Javascript |
| jsa | JRun Server Application |
| jsb | JavaScript Bean |
| jsb | JAWS Script Binary File |
| jsbundle | React Native JSBundle |
| jsc | Jeroo Source Code |
| jsc | Rhino Compiled Javascript |
| jscript | JScript Source Code |
| jscript | Windows Media Player JScript |
| jsd | JAWS Script Documentation File |
| jsdbx | ServiceNow UI Script |
| jsdtscope | Eclipse IDE JavaScript Development Tool Scope Data |
| jsfl | Flash JavaScript |
| jsh | JAWS Script Header |
| jsh | Jsh Script |
| jshtm | Visual Studio JavaScript Document |
| jsi | JAWS Personal Settings Data |
| jsl | JMP Scripting Language File |
| jslib | Unity WebGL Extension |
| jsm | JavaScript Module |
| jsm | JAWS Script Message File |
| jsp | Java Servlet |
| jspa | Java Servlet Alias |
| jspf | Java Server Page Fragment |
| jspx | Java Servlet XML |
| jss | JavaScript Style Sheet |
| jss | JAWS Script Source |
| jst | Code Crusader Symbol Database |
| jsx | Adobe ExtendScript File |
| jsxbin | Adobe ExtendScript Binary File |
| jsxinc | Adobe ExtendScript Include File |
| jtb | JavaCC Grammar File |
| jtm | JT Harness Template Data |
| jtpl | IBM Rational Application Developer for WebSphere Template Data |
| jtr | JavaTest Results Data |
| ju | JU Source Code |
| judi | JudoScript Include |
| judo | JudoScript Source Code |
| jug | Jug Code |
| jui | QT Jambi User Interface Data |
| juk | Juka Source Code |
| just | Just Script |
| jv | ICE JVCC Source Code |
| jwg | OpenScript Script |
| jwp | CloudTran Build Properties |
| jwr | Rocket LegaSuite GUI Runtime |
| jws | Java Web Service |
| jww | Java Web-Workflows data |
| jx | Cocoon JX Template |
| jxdm | MCS Markup Editor Java Server Page |
| jxs | Jitter Shader Description Data |
| jy | Jython Script |
| k7p | STEP 7 Lite Project |
| kb | Borland C++ Keyboard Script |
| kc | KeyKit Collection |
| kci | Visual Studio .NET DB Project Indexing Options |
| kcl | Kyoto Common LISP Source Code |
| kcmsf | Korean Central News Agency Website Script |
| kcode | Kano Code Artwork Data |
| kct | Keyman Developer Customisation Source File |
| kd | Koara Source Code |
| kdevdlg | KDevelop Dialog Box |
| kdevelop | KDevelop Project |
| kdevses | KDevelop Session Data |
| kdmp | Windows CE Crash Dump File |
| kds | Kodos Data |
| kenlm | KenLM Language Model Data |
| kep | Turbo Pascal Compressed Kepler Data |
| kev | QNX System Profiler Event Log |
| kfb | Pyke Knowledge Fact Base |
| kg | KAGSA Source Code |
| kis | KiScript Script |
| kit | CodeKit Data |
| kjp | KJP Source Code |
| kkm | Kkrunchy Map Data |
| kl | Haxial Compiler KL Source |
| kl1 | KLone Embedded Webserver Page |
| kl3 | KL3 Source Code |
| km3 | Kernel MetaMetaModel File |
| kmk | VirtualBox kBuild Makefile |
| kmn | Keyman Developer Keyboard Source File |
| ko | Linux 2.6 Kernel Object |
| kodu | Kodu Game |
| kodu2 | Kodu World Data |
| koe | Turbo Pascal for DOS KOE Data |
| kof | Kylix Options File |
| koi | Kokogut Module Interface File |
| kop | Kokogut Compiler Options File |
| kpp | Keyman Developer Product Installer Source File |
| kps | Keyman Developer Package Source File |
| kpz | Komodo IDE Compressed Package |
| kqb | Pyke Knowledge Question Base |
| krb | Pyke Knowledge Rule Base |
| krt | Visual FoxPro KRT File |
| ks | Mer Kickstart File |
| ksh | KSH Shell Script |
| ksql | KSQL Script |
| kss | Kinetic Style Sheet |
| kt | Kotlin Source Code |
| kts | Kotlin Script |
| ktt | Uniface Keyboard Translation Table |
| ktx | OpenGL Texture Data |
| ktx2 | Khronos 2 OpenGL Texture Data |
| kv | Kivy Source Code |
| kvh | Key-Value-Hierarchy Format Data |
| kwf | Delphi Pascal Chart Data |
| kx | KiXtart Tokenized Script |
| kxf | Koda FormDesigner Data |
| kxx | Keyman Developer Encrypted Keyboard File |
| kzb | Kanzi Binary |
| l | Lex Source Code |
| l | Xcode Core Lex Source Code |
| la | GNU Libtool Archive |
| lai | GNU Libtool Temporary Data |
| lang | GtkSourceView Language Definition |
| layerdiagram | Visual Studio Layer Diagram |
| layout | Code::Blocks Layout File |
| lba | Liberty BASIC Source Code |
| lbb | LB Booster Program |
| lbj | Learning Based Java Source Code |
| lbk | VersaPro Local Logic Block Backup |
| lbl | dBASE IV Label Data |
| lbp | ProWorx NXT Logic Pointer Data |
| lbr | Micro Focus COBOL Data |
| lbr | Omnis Library |
| lbs | Omnis Application Library |
| lbt | Visual FoxPro Label Memo |
| lbx | Visual FoxPro Label |
| lc | LiveCode Script |
| lc3 | LC-3 Assembly Source Code |
| lcf | Norton Guide Linker Control File |
| ldd | Visual Studio Logical Datacenter Diagram Data |
| ldf | ProWorx NXT Logic Data File |
| ldml | Lasso Dynamic Markup Language Document |
| lds | Binutils Linker Script |
| lds | Liquid Studio Source Code |
| ldx | Liquid Studio Compiled Executable |
| less | LESS Style Sheet |
| lfm | Lazarus Form |
| lg32 | GFA-BASIC 32 Library |
| lgf | ProWorx NXT Log Book Note |
| lgo | Logo Source Code |
| lgo | LOGO!Soft Comfort Standard Data |
| lgo | MSWLogo Program Design |
| lgt | Logtalk Source Code |
| lhs | Literate Haskell Script |
| lib | ESPL Encrypted Programming File |
| lib | Game Maker Actions Library |
| lib | Visual C++ Import Library |
| license | Visual Studio Binary License Data |
| licx | Visual Studio License Text |
| lid | Dylan Library |
| lif | HP-UX LIF File |
| lin | Valve Source Leak Pointfile |
| lin | µVision Linker Control Data |
| link | CodeWarrior Link Data |
| link | Eclipse Link File |
| links | Links Programming Language Source Code |
| lino | LinoLab Source Code |
| linq | LINQPad Source Code |
| lipg | LiPG Parser |
| lis | HP OpenVMS List Data |
| litcoffee | CoffeeScript Literate Script |
| livecode | LiveCode Stack File |
| lkc | LTIB Top Level Config |
| lkf | Code Composer Studio Linker Options Data |
| lkf | ST Visual Develop IDE Link Data |
| lkr | MPLAB Linker Data |
| lks | Lookout Direct Source Code |
| lku | ST Microelectronics OS20 Linked Unit |
| ll | Links Programming Language Template |
| ll | Low Level Virtual Machine Assembly Source Code |
| ll | Xilinx ISE ASCII Logic Allocation Data |
| llb | LabVIEW Library |
| lld | LOGO!Soft Comfort LAD Data |
| llf | IITM Local Language File |
| llp | VersaPro Compiled Local Logic Block |
| llsp | LEGO Education SPIKE Project |
| llx | LabVIEW Virtual Instrument Library Backup |