| Extension | File Type | Files scanned | Contributor |
| J | JAR Compressed Archive | 14 | |
| J | JAR Compressed Archive (with Security Envelope) | 15 | |
| JACKSUM | Jacksum fingerprints | 7 | Johann N. Loefflmann |
| JAD | Java Application Descriptor | 14 | |
| JAP | Jalbum Project | 20 | Scrapie |
| JAR | JARCS compressed archive | 4 | |
| JAR | Java Archive | 82 | |
| JAR | Mozilla Firebird theme | 17 | |
| JBF | Jasc PSP Browser Cache | 12 | Dr.Ryo(TM) |
| JBG | JBIG raster bitmap | 21 | |
| JBI | Relative Job Data | 5 | |
| JCB | CB-JCBrooke module/song | 13 | |
| JCC | Japan Crossword File (Dmitry Torshin format) | 2 | .Cryorb |
| JCE | JWPce document | 3 | |
| JCP | Jesusonic Preset | 9 | |
| JDBG | Embedded JCL debug info | 2 | |
| JDF | Job Definition Format Job File | 4 | |
| JDT | Capture Classic Filler - Accelio JetForm | 10 | |
| JDX | JCAMP Spectroscopic Data Exchange Format | 10 | |
| JEF | Janome NH10000 Sewing Machine Stitch File | 6 | FILExt |
| JG6 | BigJig Jigsaw | 4 | |
| JIF | Jeff's Image Format bitmap | 21 | |
| JIG | GameHouse Jigsaw Game | 52 | |
| JMG | 16 bit adaptive RLE compressed bitmap | 2 | |
| JNB | Sigma Plot Workbook | 9 | |
| JNG | JPEG Network Graphics bitmap | 12 | |
| JNLP | Java Web Start application descriptor | 14 | |
| JNT | Windows Journal | 22 | |
| JO | FlowJo Mac Workspace | 13 | |
| JOB | Tripod Data Systems Job file | 2 | |
| JOBOPTIONS | Acrobat Distiller Job Options | 44 | |
| JOY | Trackjoy GUS Tracker module | 2 | |
| JP2 | JPEG-2000 JP2 bitmap | 21 | |
| JP2 | LuraWave JPEG-2000 bitmap | 36 | |
| JPC | JPEG-2000 Code Stream bitmap | 22 | |
| JPG | JFIF JPEG Bitmap | 75 | |
| JPG | JFIF-EXIF JPEG Bitmap | 12 | |
| JPG | JPEG Bitmap | 77 | |
| JPG | Macintosh JPEG bitmap (MacBinary) | 1 | |
| JPX | JBuilder Project | 8 | |
| JPX | jalbum image info | 9 | |
| JR2 | Fishing Simulator 2 addon | 5 | Mamo |
| JRC | Jrchive compressed archive | 4 | |
| JRXML | JasperReports JRXML report definition | 18 | |
| JSB | JavaScript Bean file | 11 | |
| JSD | eFAX Jet Suite Document | 3 | |
| JSF | Jahshaka Scene File | 4 | |
| JTD | Ichitaro document | 26 | |
| JWC | JewelCase Maker project | 6 | |
| JWC | Jw_cad data | 8 | |
| JWL | Easy CD Creator's media label | 17 | |
| JWR | LegaSuite GUI Runtime | 6 | |
