| jsr | Job Safety Report Definition |
| jsv | Java Structure Viewer Document |
| jtbackup | Just Type Backup File |
| jtk | IBM Infoprint Manager for AIX Job Ticket Data |
| jtl | Apache JMeter Data |
| jtx | Microsoft Exchange ESE Log |
| jvxl | Jmol Voxel File |
| jwb | JIRA Workflow Data |
| jwk | JSON Web Key |
| jxe | OpenJ9 Class Loader JXE Data |
| jxt | Daqarta Juxt Data |
| jz | JamesZhu Browser Database |
| k | LS-DYNA Input Data |
| k01 | Clarion Database Keys |
| k03 | Clarion for DOS Database Key |
| k1 | DataFlex Index |
| k2 | DataFlex Index |
| k2p | Kryptel Encrypted Data |
| k3 | DataFlex Index |
| k4 | DataFlex Index |
| k5 | DataFlex Index |
| k6 | DataFlex Index |
| k7 | DCMO6 Emulator Tape Image |
| k7 | DataFlex Index |
| ka | Karma Assets Data |
| kab | Kmail Addressbook Data |
| kad | NI Kore Data |
| kal | Hondata KManager Calibration Data |
| kali | Kali Linux Related Data |
| kam | BEL Framework KAM Database |
| kan | Kwalitan Annotation Data |
| kbb | Jetico BestCrypt Key Block Backup |
| kbd | MountFocus Keyboard Designer Keyboard |
| kbk | Medeco XT Codebooks for Keys Data |
| kbp | Keirsey-Bates Temperament Sorter Profile Data |
