| Extension | Summary |
|---|
| .J | Java Source File |
| .J01 | Akeeba Multi-part Backup Archive |
| .J2C | JPEG 2000 Code Stream |
| .J2I | Jazz 2 Internet Link File |
| .J2K | JPEG 2000 Image |
| .J2L | Jazz Jackrabbit 2 Level File |
| .J3O | jMonkeyEngine 3D Scene |
| .J64 | Atari Jaguar Game ROM |
| .J6I | Ricoh Camera Image File |
| .JA | Jet Archive |
| .JAC | JaStaCry Encrypted File |
| .JAD | Java Application Descriptor File |
| .JAG | Atari Jaguar Game ROM |
| .JAM | Jam STAPL File |
| .JAP | jAlbum Project Settings File |
| .JAR | Java Archive File |
| .JAR.PACK | Pack200 Packed Jar File |
| .JARC | WiiU or 3DS Archive File |
| .JARVIS | Jarvis Subscriber File |
| .JAS | Paint Shop Pro Compressed Graphic |
| .JASPER | JasperReports Data File |
| .JAV | Java Source File |
| .JAVA | Java Source Code File |
| .JB2 | JBIG2 Image |
| .JBC | Jam STAPL Byte-Code File |
| .JBEAM | BeamNG Vehicle Definition Format |
| .JBF | Paint Shop Pro Browser Cache File |
| .JBG | Joint Bi-level Image Group File |
| .JBI | Motoman Robot Job File |
| .JBIG | Joint Bi-level Image Group File |
| .JBIG2 | JBIG2 |
| .JBK | Juno Backup File |
| .JBMP | JAmes OS Bitmap Image |
| .JBR | Paint Shop Pro Brushes File |
| .JBX | Microsoft Play It! Music File |
| .JC | FlashGet Partial Download |
| .JC! | FlashGet Incomplete Download |
| .JCD | JewelCAD Design |
| .JCEKS | JCEKS Keystore File |
| .JCL | JCloze File |
| .JCLIC | JClic Project File |
| .JCP | JCreator Project File |
| .JCR | Nonogram Puzzle File |
| .JCRYPT | JCRYPT File |
| .JCW | JCross File |
| .JCZ | Liquid Motion Animation |
| .JDAT | J&ASoft Data File |
| .JDB | Symantec Endpoint Protection Update File |
| .JDC | JDownloader Links File |
| .JDF | Adobe Acrobat Job Definition File |
| .JDIS | JD Inno Setup Configurator |
| .JDP | BlackBerry JDE Application Project |
| .JDR | IROAD Dash Cam Video File |
| .JDX | JCAMP-DX Data File |
| .JED | Xilinx JEDEC Programming File |
| .JEF | Janome Embroidery Format |
| .JEM | JSON Entity Model File |
| .JET | JET Database File |
| .JEX | Joplin Export File |
| .JFI | JPEG File Interchange Image |
| .JFIF | JPEG File Interchange Format |
| .JFPROJ | JSON Font Project File |
| .JG4 | BigJig Jigsaw Puzzle |
| .JGC | JungleTac ROM File |
| .JGCSCS | EditPad Custom Syntax Coloring Scheme File |
| .JGW | JPEG World File |
| .JGZ | Gzipped Javascript File |
| .JHH | Tetris Online Poland Game Data Archive |
| .JHTML | Java HTML Web Page |
| .JIA | Digital Photo Navigator Album |
| .JIC | JTAG Indirect Configuration File |
| .JIF | JPEG Interchange Format Image |
| .JIFF | JPEG Image File Format |
| .JIGSAW | Jigsaw Explorer Save File |
| .JIS | Japanese Industry Standard Text |
| .JKB | Jedi Knight Bot File |
| .JKM | JAWS Key Map File |
| .JKS | Java Keystore File |
| .JL | Julia Source Code File |
| .JLR | Juliar Source Code File |
| .JLS | JPEG-LS Image |
| .JMC | JM-Crypt Encrypted File |
| .JMCE | JM-Crypt Encrypted File |
| .JMCK | JM-Crypt Key File |
| .JMCP | JM-Crypt Encrypted File |
| .JMCR | JM-Crypt Encrypted File |
| .JMCX | JM-Crypt Encrypted File |
| .JMDL | WiiU or 3DS Model File |
| .JMF | Jackhammer Project |
| .JMOT | WiiU or 3DS Animation File |
| .JMP | JMP Data File |
| .JMS | Masher Project |
| .JMT | JMatch File |
| .JMV | Jnes Movie File |
| .JMX | JMix File |
| .JNB | SigmaPlot Notebook File |
| .JNG | JPEG Network Graphic |
| .JNILIB | JNI Library |
| .JNLP | Java Web Start File |
| .JNP | Java Web Start File |
| .JNR | JoinNet Recording File |
| .JNT | Windows Journal File |
| .JNX | BirdsEye JNX Raster Map |
| .JO | jo Audio File |
| .JO-7Z | jo Audio File |
| .JOB | Windows Task Scheduler Job File |
| .JOBOPTIONS | Adobe Joboptions File |
| .JOE | JOE Document |
| .JOINED | HJ-Split Data File |
| .JOY | CryENGINE Facial Editor Joystick File |
| .JP1 | Japanese (Romaji) Text File |
| .JP2 | JPEG 2000 Core Image File |
| .JPA | Akeeba Backup Archive |
| .JPC | JPEG 2000 Code Stream File |
| .JPD | Java Process Definition |
| .JPE | JPEG Image |
| .JPEG | JPEG Image |
| .JPF | JPEG 2000 Image |
| .JPG | JPEG Image |
| .JPG-LARGE | Large JPEG Image |
| .JPG2 | JPEG 2000 Image |
| .JPGW | JPEG World File |
| .JPH | JProbe Memory Snapshot |
| .JPN | Windows Japanese Noise-Word List File |
| .JPR | JBuilder Project |
| .JPS | Akeeba Backup Archive |
| .JPW | World File for JPEG |
| .JPX | JBuilder Project |
| .JQ2SAVEGAME | Jewel Quest 2 Saved Game |
| .JQ3SAVEGAME | Jewel Quest III Saved Game |
| .JQZ | JQuiz File |
| .JRC | PCjr Cartridge ROM File |
| .JRPRINT | JasperReports Print File |
| .JRS | Exchange Reserve Transaction Log File |
| .JRTF | JAmes OS Rich Text File |
| .JRXML | JasperReports Layout File |
| .JRZ | PSP Game Data File |
| .JS | JavaScript File |
| .JSD | HP JetSuite Document |
| .JSDA | Jazer Data File |
| .JSE | JScript Encoded File |
| .JSF | Project64 Joystick File |
| .JSFL | Animate JSFL Script File |
| .JSH | Jsh Script |
| .JSL | JMP Script |
| .JSON | JavaScript Object Notation File |
| .JSONL | JSON Lines File |
| .JSONLZ4 | Compressed Firefox User Profile Data File |
| .JSP | Java Server Page |
| .JSPA | Java Servlet Alias |
| .JSPF | Java Server Page Fragment |
| .JSPX | XML Java Server Page |
| .JSS | JAWS Script |
| .JST | JavaServer Page Document |
| .JSX | React JavaScript File |
| .JSXBIN | Binary ExtendScript Script File |
| .JSXINC | ExtendScript Included Script File |
| .JT | JT Open CAD File |
| .JTBACKUP | Just Type Backup File |
| .JTD | JustSystems Ichitaro Document |
| .JTEX | WiiU or 3DS Texture File |
| .JTF | JPEG Tagged Interchange Format |
| .JTP | Windows Journal Template |
| .JTS | Cyberlink AVCHD Video File |
| .JTV | JRiver TV File |
| .JTX | ESE Transaction Log |
| .JUDE | JUDE Project File |
| .JUK | Juka Source Code File |
| .JVS | JavaScript Proxy Autoconfig File |
| .JVSG | Video Surveillance Project |
| .JVSGZ | Compressed Video Surveillance Project |
| .JWC | JewelCase Maker File |
| .JWL | Roxio Jewel Case File |
| .JWPUB | Jehovah's Witnesses Publication |
| .JWS | Java Workspace Settings File |
| .JXL | JPEG XL Image |
| .JXR | JPEG XR Image |
