| jqz | JQuiz Data File |
| jr6 | AGICO JR-6 Spinner Data |
| jra | AGICO JR-5 Spinner Data |
| jrb | OpenNMS JRobin Database |
| jrc | PCjr Cartridge ROM Image |
| jrd | OpenNMS JRobin Database |
| jrm | IBM Tivoli Resource Model Builder Data |
| jrmime | HDTracks Downloader Data |
| jrn | SAP POS Journal File |
| jrs | Microsoft Exchange Reserved Transaction Log |
| jrx | MDI DataScan Task List |
| jsd | Jazer Data File |
| jsda | Jazer Data File |
| jsee | EmEditor Recorded Macro |
| jsgf | Java Speech Grammar Format Data |
| jsk | JetAudio Skin |
| jsml | JSim Markup Language Data |
| jsn | JetSoft Shield Now Data |
| json | JavaScript Object Notation |
| jsonld | JSON-LD Data |
| jsonp | JSON with Padding Data |
| 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 |
| jus | Jusan Fidelity License Data |
| jvdb | The Journal Volume Database |
| jvsk | jetVideo Skin Data |
| jvxl | Jmol Voxel File |
| jwb | JIRA Workflow Data |
| jwk | JSON Web Key |
| jwlibrary | JW Library Data |
| jwpub | JW Library Publication |