| .j | Java Source Code |
| .j2b | Jazz Jackrabbit 2 Music |
| .j2c | JPEG2000 Image Format |
| .j2d | Jazz Jackrabbit Data |
| .j2e | Jazz Jackrabbit 2 Episode |
| .j2i | Jazz Jackrabbit 2 Internet Server Link Format |
| .j2k | JPEG2000 Code Stream Image |
| .j2l | Jazz Jackrabbit 2 Level Format |
| .j2si | Jazz Jackrabbit 2 Internet Server Link |
| .j2w | ArcExplorer World |
| .j3o | JMonkeyEngine 3D Scene |
| .j6i | Ricoh Digital Camera Image |
| .ja | Jet Archive |
| .jac | JaStaCry Encrypted Format |
| .jacl | IBM WebSphere Server Script |
| .jad | Java Application Descriptor Format |
| .jade | Jade Programming Language Source Code |
| .jag | Jagex Data Format |
| .jai | Jaikoz Settings |
| .jam | Leisure Suit Larry Magna Cum Laude Archive Format |
| .jan | Janome Digitizer Embroidery Object |
| .jap | Japanese Nintendo 64 Game ROM |
| .jar | Compressed Archive Format Package For Java Classes And Data Format |
| .jar.pack | Oracle Pack200 Packed JAR Format |
| .jardesc | Eclipse JAR Settings |
| .jarvis | Jarvis Subscriber Format |
| .jas | Corel PaintShop Photo Pro Bitmap Graphics Format |
| .jasper | JasperReports Data Format |
| .jav | Java Language Source Code Format |
| .java | Java Language Source Code Format |
| .javajet | Eclipse JET Java Template |
| .jax | Java Executable JAR |
| .jb2 | JBIG2 Image Format |
| .jbc | Jam STAPL Byte-Code Format |
| .jbf | Corel PaintShop Photo Pro Image Browser Format |
| .jbg | Image JBIG 1-bit Raster |
| .jbi | Motoman Robot Job Format |
| .jbig | JBIG Raster Image Graphics |
| .jbig2 | JBIG2 |
| .jbk | Juno Backup Format |
| .jbl | Doom Scene Add-on |
| .jbmp | JAmes OS Bitmap Image |
| .jbr | Corel PaintShop Photo Pro Brushes Format |
| .jbt | JAWS |
| .jc | FlashGet Partial Download Format |
| .jc! | FlashGet Incomplete Download Format |
| .jcc | Code Crusader Project Data |
| .jccfg3 | FlashGet Incomplete Download |
| .jcd | FlashGet Saved |
| .jcf | JAWS Configuration |
| .jclic | JClic Project Format |
| .jcm | Java Commerce Message |
| .jcp | Jesusonic Preset Format |
| .jcrypt | JCRYPT Encrypted Format |
| .jcs | Java Control Source |
| .jcsk | JetCast Skin |
| .jct | JCT |
| .jcz | Microsoft Liquid Motion Animation Format |
| .jdb | Norton Antivirus Definiton Update File Format |
| .jdc | JDownloader Links Format |
| .jdf | Adobe Acrobat Job Definition Format |
| .jdp | BlackBerry JDE |
| .jdw | BlackBerry JDE |
| .jdx | JSpell Dictionary |
| .jeb | PENSIM Output |
| .jed | Xilinx Integrated Software Environment Device Configuration Information Format |
| .jef | Janome Embroidery Layout Format |
| .jetinc | Eclipse IDE Template Fragment |
| .jewelquestsavedgame | Jewel Quest Saved Game |
| .jfd | JAWS Synopsis |
| .jff | JAWS Ini |
| .jffs2 | Journalling Flash File System 2 Disk Image |
| .jfi | JPEG File Interchange Format |
| .jfif | JPEG Interchange Format |
| .jfl | SigmaPlot |
| .jfr | WJVN Fragment |
| .jfs | Linux/Unix Journaling System |
| .jfx | J2 Fax |
| .jg4 | BigJig Jigsaw Puzzle Format |
| .jgcscs | EditPad Syntax Scheme |
| .jgd | Corel PaintShop Photo Pro Gradients |
| .jgf | JAWS Ini |
| .jgp | QWS3270 PLUS Session Parameter |
| .jgs | QWS3270 PLUS Script |
| .jgw | JPEG World Format |
| .jgz | Gzipped JavaScript Format |
| .jhm | Java Help Mapping |
| .jhtml | ATG Dynamo Server Page (Java HTML) |
| .jia | Digital Photo Navigator Album |
| .jic | Java Icon Format |
| .jif | JPEG Interchange Format |
| .jiff | JPG Bitmap Image |
| .jig | GameHouse Puzzle Jigsaw Game |
| .jigsaw | Jigsaw Explorer Puzzle Format |
| .jigz | Jiggsaw Puzzle |
| .jis | Japanese Industry Standard Text Format |
| .jisp | Tkabber Emoticon Set |
| .jit | Aika Online Texture |
| .jj | Doodle C64 Compressed Image Format |
| .jjf | Flash Jester Jugglor |
| .jjjs | Claris Organizer Format |
| .jkb | Quake 3 Arena Bot AI |
| .jkl | Jedi Knight |
| .jkm | JAWS Ini Configuration Format |
| .jks | Jedi Knight Save |
| .jl | Julia Source Code Format |
| .jlb | PAC Mate BX Keyboard Driver |
| .jlc | Byte Compiled Lisp-based Script |
| .jls | JAWS |
| .jma | NSRT Compressed Game |
| .jmd | Corel PaintShop Pro Image Map Settings |
| .jmf | Java Media Framework |
| .jml | Java Modeling Language Specification |
| .jmp | SAS JMP Discovery Chart-to-statistics |
| .jms | Voxtron TeleButler Interactive Voice Response Call Flow |
| .jmv | Jnes Record Format |
| .jmx | Apache JMeter |
| .jnb | SigmaPlot Workbook |
| .jng | JPEG Network Graphic Format |
| .jnilib | Mac OS X Java JNI Library Format |
| .jnl | Ingres Journal |
| .jnlp | Java Network Launching Protocol Format |
| .jnt | Microsoft Windows Journal Note Format |
| .jnx | Garmin Raster Map |
| .jo | Jo Audio Format |
| .jo-7z | Jo Audio Format |
| .job | Microsoft Windows Task Scheduler Task Object Format |
| .joboptions | Adobe Acrobat Job Options Format |
| .jobs | VirtualDub Job |
| .jod | Microsoft Jet OLEDB.4.0 |
| .joe | .joe Editor Document Format |
| .joined | HJSplit Data Format |
| .joml | JOOB Object Modelling Language |
| .jomproj | Microsoft Visual Studio JOOB Object Model Project |
| .jor | Microsoft SQL Server Journal |
| .jot | Jot+ Notes Note Data |
| .journal.doc | SolidWorks Design Journal Template |
| .journal_info_block | Apple Mac OS Xsystem |
| .joy | CryENGINE Facial Editor Joystick Settings Format |
| .jp1 | Japanese (Romaji) Text Format |
| .jp2 | JPEG 2000 Image Format (ISO 15444-1) Format |
| .jp3 | Electric Quilt |
| .jp4 | Electric Quilt |
| .jp5 | Electric Quilt |
| .jpa | Joomla Installation Backup |
| .jpage | Eclipse Java Scrapbook Page |
| .jpb | Publish Manager Report |
| .jpc | JPEG2000 Code Stream Image Format |
| .jpd | Joint PhotoDefiner Image Format |
| .jpe | JPEG Bitmap Image Format |
| .jpeg | JPEG Bitmap Image Format |
| .jpegenx | Egis Encrypted JPEG |
| .jpeg_160x120 | 160 X 120 Pixels JPEG Image |
| .jpeg_170x220 | 170 X 220 Pixels JPEG Image |
| .jpf | JPEG 2000 Extended Image Format (ISO 15444-2) Format |
| .jpg | JPEG Bitmap Image Format |
| .jpg2 | JPEG 2000 Image Format (ISO 15444-1) Format |
| .jpgenx | Egis Encrypted JPG |
| .jpgw | JPEG World Format |
| .jpgx | Egis Encrypted JPG Bitmap Image |
| .jpg_108x192 | Android Picture |
| .jpg_128x128 | Android Picture |
| .jpg_128x160 | Android Picture |
| .jpg_128x96 | Android Picture |
| .jpg_160x120 | Android Picture |
| .jpg_160x128 | Android Picture |
| .jpg_170x128 | Android Picture |
| .jpg_170x220 | Android Picture |
| .jpg_220x176 | Android Picture |
| .jpg_240x320 | Android Picture |
| .jpg_320x240 | Android Picture |
| .jpg_320x320 | Android Picture |
| .jpg_480x320 | Android Picture |
| .jpg_512x512 | Android Picture |
| .jpg_56x42 | Android Picture |
| .jpg_t | JPEG Image |
| .jph | JProbe Memory Snapshot Format |
| .jpi | Jupiter Encrypted XML |
| .jpl | JProbe Settings |
| .jpm | JPEG 2000 Multi-layer Image Format (ISO 15444-6) |
| .jpn | Windows Japanese Noise-Word List Format |
| .jpp | Jigsaw Puzzle Player Puzzle Definition |
| .jpr | Oracle JDeveloper Model Project |
| .jps | Stereospic JPEG Format |
| .jpt | Jprobe Threadalyzer Memory Snapshot |
| .jpw | ESRI JPEG World Format |
| .jpx | JPEG 2000 Extended Image Format (ISO 15444-2) Format |
| .jpz | Puzzle Solver Puzzle |
| .JQ3SaveGame | Jewel Quest III Saved Game |
| .jqx | Microsoft Windows Help Related |
| .jrc | PCjr Cartridge ROM Format |
| .jrf | The Jumbot Bot Level Navigation |
| .jrl | BioWare Aurora Game Engine Journal |
| .jrprint | JasperReports Print Format |
| .jrs | Extensible Storage Engine (ESE) Reserve Transaction Log Format |
| .jrtf | JAmes OS Rich Text Format |
| .jrxml | JasperReports Layout Format |
| .jrz | Sony Playstation Portable Game Data Format |
| .js | JavaScript Format |
| .jsa | JRun Server Application |
| .jsb | JAWS Script Binary |
| .jsc | JavaScript Config |
| .jsd | Jazer Financial Format |
| .jsda | Jazer Financial Format |
| .jse | JScript Encoded Script Format |
| .jsf | Adobe Fireworks Batch Script |
| .jsfl | Flash JavaScript Format |
| .jsh | JAWS Script Header Format |
| .jsi | JAWS Personalized Settings |
| .jsk | JetAudio |
| .jsl | Corel PaintShop Photo Pro Shapes |
| .jsm | JAWS Script Message |
| .jsml | JSim Model |
| .jsn | Shield Now Shield |
| .jso | Compressed ISO Image |
| .json | JavaScript Object Notation Format |
| .jsonp | JSON-with-padding Source Code |
| .jsp | JAVA Server Page Format |
| .jspa | Java Servlet Alias Format |
| .jspf | Java JSP Fragment |
| .jspx | XML Java Server Page |
| .jsr | Job Safety Report Definition |
| .jss | JAWS Script Source Format |
| .jst | Java Server Page |
| .jsv | Java Structure Viewer |
| .jsx | Adobe After Effects Script Format |
| .jsxbin | Binary JavaScript Format |
| .jsxinc | ExtendScript Included Script Format |
| .jt | JT Open CAD Format |
| .jtb | JTB JavaCC Grammar |
| .jtbackup | Just Type Notes Backup Format |
| .jtd | Ichitaro Document |
| .jtdc | Ichitaro Compressed Document |
| .jtf | JPEG Tagged Interchange Format Image |
| .jtg | Joint Test Action Group Configuration |
| .jtk | Sun Microsystems Java Toolkit |
| .jtl | JMeter Test Results |
| .jtp | Microsoft Windows Journal Template Format |
| .jtpl | Rational Application Developer WebContent Theme Template |
| .jts | JVC Cyberlink AVCHD Video Format |
| .jtt | Ichitaro Document Template |
| .jtv | JRiver Media Center Recorded TV Format |
| .jtx | Jrju Text Format |
| .jude | JUDE Project Format |
| .jvs | Web Proxy Autodiscovery Protocol Format |
| .jvsg | Video Surveillance Project |
| .jvt | JRiver Recorded Video |
| .jw | Microsoft WordPad |
| .jwc | JewelCase Maker Saved |
| .jwg | Java |
| .jwl | Roxio Jewel Case Format |
| .jwr | LegaSuite GUI J Walk Runtime |
| .jws | Oracle WebLogic Server Java Web Services Format |
| .jxr | JPEG XR Image |
| .jz | JamesZhu Internet Browser Database |
