| 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 |
| jtr | JavaTest Results Data |
| ju | JU Source Code |
| judo | JudoScript Source Code |
| jug | Jug Code |
| jui | QT Jambi User Interface Data |
| jv | ICE JVCC Source Code |
| jwg | OpenScript Script |
| jwp | CloudTran Build Properties |
| jwr | Rocket LegaSuite GUI Runtime |
| jws | Java Web Service |
| jx | Cocoon JX Template |
| jy | Jython Script |
| k7p | STEP 7 Lite Project |
| kb | Borland C++ Keyboard Script |
| kc | KeyKit Collection |
| kci | Visual Studio .NET DB Project Indexing Options |
| kct | Keyman Developer Customisation Source File |
| kd | Koara Source Code |
| kdevdlg | KDevelop Dialog Box |
| kdevelop | KDevelop Project |
| kdevprj | KDevelop Project |
| kdmp | Windows CE Crash Dump File |
| kev | QNX System Profiler Event Log |
| kfb | Pyke Knowledge Fact Base |
| kgi | Kokogut Module Interface File |
| kjp | KJP Source Code |
| kkm | Kkrunchy Map Data |
| kl | Haxial Compiler KL Source |
| kl3 | KL3 Source Code |
