| Abbreviation | Definition |
|---|---|
| JET | Hybrid JetFax Fax file |
| JFF | JPEG File Interchange Format Bitmap graphics |
| JFI | JPEG File Interchange Format Bitmap graphics |
| JFIF | JPEG File Interchange Format Bitmap graphics |
| JIF | Bitmap graphics (JPEG File Interchange Format) |
| JIFF | JPEG Image File Format |
| JMF | Jello Music File |
| JMS | UltraForce JMusic file - Music format |
| JN1 | Epic MegaGames Jill of the Jungle Game data |
| JNG | JPEG Network Graphics |
| JOB | Job object (Scheduler) |
| JOI | Join conferences file (XBoard) |
| JOR | Journal file (Gupta SQLWindows) |
| JOU | Journal file (VAX Edt editor) |
| JPC | Bitmap graphics (Japan Picture format) |
| JPE | Bitmap graphics (JPEG Joint Photography Experts Group format) |
| JPEG | Joint Photographic Experts Group (see JPG) |
| JPEG-LS | Joint Photographic Experts Group Lossless |
| JRC | Compressed file archive (JRchive) |
| JRF | Jumbot Recognition File |