| jtl | Apache JMeter Data |
| jtx | Microsoft Exchange ESE Log |
| jus | Jusan Fidelity License Data |
| jvdb | The Journal Volume Database |
| jvxl | Jmol Voxel File |
| jwb | JIRA Workflow Data |
| jwk | JSON Web Key |
| jwlibrary | JW Library Data |
| jwpub | JW Library Publication |
| jwt | JSON Web Token |
| jxe | OpenJ9 Class Loader JXE Data |
| jxl | TBC JobXML Data |
| jxt | Daqarta Juxt Data |
| jz | JamesZhu Browser Database |
| jzt | JaZwa Binary Translator Data |
| k | LS-DYNA Input Data |
| k01 | Clarion Database Keys |
| k03 | Clarion for DOS Database Key |
| k0i | k0-INAA Data |
| k1 | DataFlex Index |
| k10 | DataFlex Index |
| k11 | DataFlex Index |
| k12 | DataFlex Index |
| k2 | DataFlex Index |
| k2p | Kryptel Encrypted Data |
| k3 | DataFlex Index |
| k4 | DataFlex Index |
| k5 | DataFlex Index |
| k6 | DataFlex Index |
| k7 | DataFlex Index |
| k7 | DCMO6 Emulator Tape Image |
| k8 | DataFlex Index |
| k9 | DataFlex Index |
| ka | Karma Assets Data |
| kab | Kmail Addressbook Data |