| jtx | Microsoft Exchange ESE Log |
| jus | Jusan Fidelity License Data |
| jvdb | The Journal Volume Database |
| jvl | Jalview Launch Data |
| jvsk | jetVideo Skin Data |
| jvwr | JPEGView Creator Data |
| jvxl | Jmol Voxel File |
| jwb | JIRA Workflow Data |
| jwk | JSON Web Key |
| jwl | JustWrite Library Data |
| 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 |
| k | MacroModel Chemical Modeller Output |
| 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 |
| k13 | DataFlex Index |
| k14 | DataFlex Index |
| k15 | DataFlex Index |
