| jef | Janome Embroidery Data |
| jeff | Java Explanation Facility Framework Data |
| jem | Optifine JSON Entity Model |
| jet | California JET Data |
| jfd | FormWare Job Definition |
| jft | JetForm Job Ticket |
| jga | John GBC Saved State |
| jgd | JVC Projectors Gamma Curve Data |
| jgd | Corel Paint Shop Pro Gradients Data |
| jgl | QWS3270 PLUS Session Layout |
| jgp | QWS3270 PLUS Session Parameter File |
| jgs | QWS3270 PLUS Script File |
| jhd | WCAD01 Job Information Data |
| jhm | JavaHelp Map |
| jia | Digital Photo Navigator Album |
| jib | Sam2p JIB File |
| jjf | FlashJester Jugglor Data |
| jks | Java Key Store |
| jlb | JDiveLog Logbook File |
| jlk | JET 3270 License Key Data |
| jlm | ILOG JViews License Manager Keys Data |
| jlp | Jumplist Extender Data |
| jlr | JetAudio Lyric Data |
| jlt | HP SmartStream Print |
| jm3 | BlastMetriX 3D Data |
| jmf | Job Messaging Format File |
| jmp | JMP Data |
| jmt | JMatch Data |
| jmx | JMix Data |
| jnl | WinDev Log |
| jnl | Ingres Journal Data |
| jns | JANIS XML Data |
| job | USoft Job Definitions Data |
| job | Windows Task Scheduler Job Object |
| jobs | VirtualDub Jobs Data |