| J62 | Ricoh camera format file |
| JAD | Java Application Descriptor file |
| JAR | Java ARchive (a compressed file containing a package of Java application-related files) |
| JAS | Graphic (Generic) |
| JAVA | Source code (Java) |
| JBD | Datafile (SigmaScan) |
| JBF | Image browser file (Paint Shop Pro) |
| JBX | Project file (Project Scheduler 4.0) |
| JCP | Java Community Process file |
| JCW | Java Callable Wrapper file |
| JDF | Job Description File (proposed XML-based standard for end-to-end job ticket specification) |
| JFF | JPEG image |
| JFIF | JPEG image |
| JIF | JPEG image |
| JMG | Laservision projection graphics file |
| JMP | Discovery chart-to-statistics (SAS JMP) |
| JN1 | Jill of the Jungle data (Epic MegaGames) |
| JNB | Workbook file (Sigma Plot 5) |
| JNT | Microsoft Windows Journal notes |
| JOB | Vector graphics file created by conversion of a IMG file (QuestVision) |
| JOR | Journal (Microsoft SQL Server) |
| JOU | Journal backup (VAX Edt editor) |
| JPC | Graphic (Japan PIC) |
| JPE | JPEG image |
| JPEG | JPEG compressed bitmap |
| JPG | JPEG bitmap |
| JPG2 | JPEG 2000 image (bitmap) file |
| JS | JavaScript source code |
| JSD | Jet Suite document (eFAX) |
| JSM | Gray Design Associates Jigsaws Galore Jigsaw File |
| JSP | A HTML page containing a reference to a Java servlet |
| JT | Open CAD file |
| JTF | Fax document (Hayes JT Fax) |
| JTK | Java ToolKit file (Sun Microsystems) |
| JTP | Windows Journal templates |
| JW | Text document (JustWrite) |
| JWL | Library (JustWrite) |
| JXX | C++ Header File |
| JZZ | Spreadsheet (Jazz) |
