| jrm | IBM Tivoli Resource Model Builder Data |
| jrn | SAP POS Journal File |
| jrs | Microsoft Exchange Reserved Transaction Log |
| jrx | MDI DataScan Task List |
| jsd | Jazer Data File |
| jsda | Jazer Data |
| jsda | Jazer Data File |
| jsk | JetAudio Skin |
| jsml | JSim Markup Language Data |
| jsn | JetSoft Shield Now Data |
| json | JavaScript Object Notation |
| 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 |
| k1 | DataFlex Index |
| k2 | DataFlex Index |
| k2p | Kryptel Encrypted Data |
| k3 | DataFlex Index |
| k4 | DataFlex Index |
