| journal | Nuance Equitrac Cache |
| joz | JOSM Session Data |
| jp | Java Japanese Font Properties Data |
| jperf | JPerf Data |
| jpgenx | Egis Encrypted JPEG Data |
| jpgx | Egis Encrypted JPEG Data |
| jpi | Jupiter Filled Forms Data |
| jpm | Optifine JSON Part Model |
| jpn | Windows Japanese Noise-word List |
| jpo | JPO Data |
| jprint | JasperReports Print Object |
| jpz | Crossword Puzzle Solver Data |
| jqz | JQuiz Data File |
| 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 |