| 1 | J | Oracle | Java Source Format |
| 2 | J2C | Joint Photographic Experts Group | JPEG 2000 Code Stream |
| 3 | J2D | Epic Games, Inc. | Jazz Jackrabbit Data |
| 4 | J2K | Joint Photographic Experts Group | JPEG 2000 Image |
| 5 | J2W | Esri | ArcExplorer World |
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| 6 | J3D | N/A | Generic |
| 7 | JA | N3V Games | Jet Archive |
| 8 | JACL | IBM | IBM WebSphere Server Script |
| 9 | JAD | Oracle | Java Application Descriptor Format |
| 10 | JADE | N/A | Jade Programming Language Source Code |
| 11 | JAM | Sierra Entertainment | Leisure Suit Larry Magna Cum Laude Archive Format |
| 12 | JAN | Janome Sewing Machine Co., Ltd. | Janome Digitizer Embroidery Object |
| 13 | JAR | Oracle | Java Archive Format |
| 14 | JAV | Oracle | Java Source Format |
| 15 | JAVA | Oracle | Java Source Code Format |
| 16 | JB2 | Joint Bi-level Image Group | JBIG2 Image |
| 17 | JBEAM | N/A | BeamNG Vehicle Definition Format |
| 18 | JBF | Corel | Paint Shop Pro Browser Cache Format |
| 19 | JBIG | Joint Bi-level Image Group | Joint Bi-level Image Group Format |
| 20 | JBR | Corel | Paint Shop Pro Brushes Format |
| 21 | JDB | Symantec | Symantec Endpoint Protection Update Format |
| 22 | JEB | N/A | PENSIM Output |
| 23 | JEF | Janome | Janome Embroidery Format |
| 24 | JFFS2 | Various Linux developers | Journalling Flash Format System 2 Disk Image |
| 25 | JFI | Joint Photographic Experts Group | JPEG Format Interchange Image |
| 26 | JFIF | N/A | JPEG Format Interchange Format |
| 27 | JFS | N/A | Linux/Unix Journaling System |
| 28 | JGS | Jolly Giant Software Inc. | QWS3270 PLUS Script |
| 29 | JGW | ESRI | JPEG World Format |
| 30 | JGWX | N/A | Blender GIS world data |
| 31 | JGZ | N/A | Gzipped Javascript Format |
| 32 | JHTML | N/A | Java HTML Web Page |
| 33 | JI | N/A | Join-Calculus interface source code |
| 34 | JIC | N/A | Java Icon Format |
| 35 | JIF | Joint Photographic Experts Group | JPEG Image Format |
| 36 | JIS | N/A | Japanese Industry Standard Text |
| 37 | JKS | LucasArts Entertainment Company, LLC | Jedi Knight Save |
| 38 | JMA | N/A | NSRT Compressed Game |
| 39 | JML | N/A | Java Modeling Language Specification |
| 40 | JNG | N/A | JPEG Network Graphic |
| 41 | JNL | Actian Corporation | Ingres Journal |
| 42 | JNLP | Oracle | Java Web Start Format |
| 43 | JNX | N/A | Garmin Raster Map |
| 44 | JOB | Microsoft | Windows Task Scheduler Job Format |
| 45 | JOBOPTIONS | Adobe Systems | Adobe Joboptions Format |
| 46 | JOURNAL_INFO_BLOCK | Apple, Inc. | Apple Mac OS Xsystem |
| 47 | JP1 | N/A | Japanese (Romaji) Text Format |
| 48 | JP2 | Joint Photographic Experts Group | JPEG 2000 Core Image Format |
| 49 | JPA | Akeeba | Joomla Installation Backup |
| 50 | JPC | Joint Photographic Experts Group | JPEG 2000 Code Stream Format |
| 51 | JPD | N/A | Java Process Definition |
| 52 | JPE | Joint Photographic Experts Group | JPEG Image |
| 53 | JPEG | Joint Photographic Experts Group | JPEG Image |
| 54 | JPF | Joint Photographic Experts Group | JPEG 2000 Image |
| 55 | JPG | Joint Photographic Experts Group | JPEG Image |
| 56 | JPG-LARGE | N/A | Twitter Large-sized JPEG Image |
| 57 | JPG-ORIGINAL | N/A | JPEG picture image |
| 58 | JPG2 | Joint Photographic Experts Group | JPEG 2000 Image |
| 59 | JPGE | N/A | Mistyped JPEG bitmap image |
| 60 | JPGENX | Egis Technology Inc. | Egis Encrypted JPG |
| 61 | JPGW | ESRI | JPEG World Format |
| 62 | JPG_T | N/A | JPEG Image |
| 63 | JPM | N/A | JPEG 2000 Multi-layer Image Format (ISO 15444-6) |
| 64 | JPS | N/A | Stereo JPEG Image |
| 65 | JPW | ESRI | World Format For JPEG |
| 66 | JPX | Joint Photographic Experts Group | JPEG 2000 Image Format |
| 67 | JRS | Microsoft | Exchange Reserve Transaction Log Format |
| 68 | JS | N/A | JavaScript Format |
| 69 | JSA | Adobe Systems Incorporated | JRun Server Application |
| 70 | JSC | N/A | JavaScript Config |
| 71 | JSCRIPT | N/A | Microsoft JScript source code |
| 72 | JSE | Microsoft | JScript Encoded Format |
| 73 | JSF | Adobe | Adobe Fireworks Batch Script |
| 74 | JSFL | Adobe Systems | Flash JavaScript Format |
| 75 | JSM | Freedom Scientific, Inc. | JAWS Script Message |
| 76 | JSON | N/A | JavaScript Object Notation Format |
| 77 | JSONLZ4 | Mozilla | Compressed Firefox User Profile Data File |
| 78 | JSONP | N/A | JSON-with-padding Source Code |
| 79 | JSP | Oracle | Java Server Page |
| 80 | JSPA | N/A | Java Servlet Alias |
| 81 | JSPX | Eclipse Foundation | XML Java Server Page |
| 82 | JST | Oracle | JavaServer Page Document |
| 83 | JSX | Adobe Systems | ExtendScript Script Format |
| 84 | JSXBIN | Adobe Systems | Binary ExtendScript Script Format |
| 85 | JSXINC | Adobe Systems | ExtendScript Included Script Format |
| 86 | JT | Siemens PLM Software | JT Open CAD Format |
| 87 | JTX | Microsoft | ESE Transaction Log |
| 88 | JU | N/A | JU source code |
| 89 | JUDO | N/A | JudoScript source code |
| 90 | JW | N/A | Microsoft WordPad |
| 91 | JWL | Roxio | Roxio Jewel Case Format |
| 92 | JXR | Microsoft | JPEG XR Image |
