| jmx | JMix Data |
| jnl | WinDev Log |
| jnl | Ingres Journal Data |
| jns | JANIS XML Data |
| job | USoft Job Definitions Data |
| job | Windows Task Scheduler Job Object |
| jobs | VirtualDub Jobs Data |
| jod | Microsoft Jet OLEDB |
| joda | Joost Open Distribution Archive |
| jof | SmartCAM Job Operations File |
| joh | Zero 88 Jester ML Update Help Data |
| joined | HJSplit Data |
| joml | JOOB Object Modelling Language Data |
| jos | Zero 88 Jester ML Software Update Data |
| jou | MSC Patran Journal |
| journal | Nuance Equitrac Cache |
| joz | JOSM Session Data |
| jp | Java Japanese Font Properties 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 |