| ixy | ISIS v2 Schematic Data |
| j3 | jBASE J3 Data |
| j3d | Atari Jaguar 3D Object |
| j3md | JMonkey Engine Material data |
| j4 | jBASE J4 Data |
| j64 | Virtual Jaguar ROM Image |
| jacksum | Jacksum Checksum Data |
| jaf | Just A Frequency Logger Backup Data |
| jam | Lightspeed Audio Lab JAM Client Data |
| jammix | Jammix Enhancer Music Transformation Data |
| jan | Janome Embroidery Design |
| jap | Nintendo 64 Japanese ROM Image |
| jaskin | jAlbum Skin |
| jasp | JASP Data |
| jbb | JobEditor32 Binary Job Data |
| jbb | Jbead Data |
| jbc | Jetico BestCrypt Container |
| jbd | SigmaScan Data |
| jbeam | BeamNG.drive Vehicle Definition Format Data |
| jbf | Paint Shop Pro Browser Cache |
| jbi | Motoman Robot Job |
| jbk | Juno Client Software Backup |
| jbm | Jbmgen Data |
| jbt | JAWS Braille Tables Data |
| jbx | Project Scheduler JBX Data |
| jbx | Music Ace 2 Jukebox Data |
| jc | FlashGet Partial Download |
| jc | CelLab Compiled Rule |
| jc1 | FlashGet Partial Download |
| jcamp | JCAMP-DX Format Data |
| jcd | FlashGet Download Database |
| jceks | Java Cryptography Extension KeyStore |
| jch | CelLab Population History Log |
| jcn | JobClock.NET TimeSummit Data |
| jcp | CelLab Pattern Data |