| .J | Oracle Corporation | Developer Files | Java Source File | Internet Explorer | Microsoft Corporation |
| .JAV | Oracle Corporation | Developer Files | Java Source Code | Microsoft Notepad | Microsoft Corporation |
| .JAVA | Oracle Corporation | Developer Files | Java Source Code | Microsoft Notepad | Microsoft Corporation |
| .JCK | Microsoft Corporation | Data Files | Microsoft Liquid Motion JACK Scene Escription | Liquid Motion | Microsoft Corporation |
| .JET | Microsoft Corporation | Database Files | JET Database File | Microsoft Access | Microsoft Corporation |
| .JFI | Joint Photographic Experts Group | Raster Image Files | JPEG File Interchange Image | Windows Photo Gallery | Microsoft Corporation |
| .JFIF | Unknown Developer | Raster Image Files | JPEG File Interchange Format | Internet Explorer | Microsoft Corporation |
| .JFW | ESRI | GIS Files | JPEG World File | Microsoft Notepad | Microsoft Corporation |
| .JGW | ESRI | GIS Files | JPEG World File | Microsoft Notepad | Microsoft Corporation |
| .JHTML | Oracle Corporation | Web Files | Java HTML Web Page | Internet Explorer | Microsoft Corporation |
| .JIF | Joint Photographic Experts Group | Raster Image Files | JPEG Image | Microsoft Windows Photos | Microsoft Corporation |
| .JLB | Microsoft Corporation | System Files | PAC Mate BX Keyboard Driver | Windows Mobile | Microsoft Corporation |
| .JMH | Unknown Developer | Raster Image Files | JPEG File Interchange Format | Internet Explorer | Microsoft Corporation |
| .JNLP | Oracle Corporation | Web Files | Java Web Start File | Microsoft Notepad | Microsoft Corporation |
| .JNT | Microsoft Corporation | Data Files | Windows Journal File | Microsoft Windows Journal | Microsoft Corporation |
| .JOB | Metamation | CAD Files | MetaCAM Nest Job File | Windows Task Scheduler | Microsoft Corporation |
| .JOR | Microsoft Corporation | Uncommon Files | Microsoft SQL Server Journal File | Microsoft SQL Server | Microsoft Corporation |
| .JP2 | Joint Photographic Experts Group | Raster Image Files | JPEG 2000 Core Image File | ACD Systems Canvas | Microsoft Corporation |
| .JPEG | Joint Photographic Experts Group | Raster Image Files | JPEG Image | Microsoft Windows Photos | Microsoft Corporation |
| .JPG | Joint Photographic Experts Group | Raster Image Files | JPEG Image | Microsoft Windows Photos | Microsoft Corporation |
| .JPG-LARGE | Zoner Software | Raster Image Files | Large JPEG Image | Internet Explorer | Microsoft Corporation |
| .JPN | Microsoft Corporation | System Files | Windows Japanese Noise-Word List File | Microsoft Notepad | Microsoft Corporation |
| .JPR | Microsoft Corporation | Text Files | WordPad Document | Microsoft WordPad | Microsoft Corporation |
| .JQX | Microsoft Corporation | Data Files | Microsoft Windows Help-Related File | Microsoft Windows | Microsoft Corporation |
| .JRS | Microsoft Corporation | Backup Files | Extensible Storage Engine (ESE) Reserve Transaction Log | Microsoft Exchange Server | Microsoft Corporation |
| .JS | NetScape | Web Files | JavaScript File | Microsoft Edge | Microsoft Corporation |
| .JSCRIPT | NetScape | Web Files | JavaScript File | Microsoft Edge | Microsoft Corporation |
| .JSE | Microsoft Corporation | Executable Files | JScript Encoded File | Microsoft Visual Studio | Microsoft Corporation |
| .JSON | Mozilla | Backup Files | Mozilla Firefox Bookmarks Backup | Microsoft Notepad | Microsoft Corporation |
| .JSONLD | Microsoft Corporation | Data Files | Microsoft Visual Studio Data File | Microsoft Visual Studio | Microsoft Corporation |
| .JSP | Oracle Corporation | Web Files | Java Server Page | Microsoft Edge | Microsoft Corporation |
| .JSPA | Xinox Software | Web Files | Java Servlet Alias | Internet Explorer | Microsoft Corporation |
| .JSPF | Oracle Corporation | Developer Files | Java Server Page Fragment | Microsoft Notepad | Microsoft Corporation |
| .JSPROJ | Microsoft Corporation | Data Files | Microsoft Visual Studio Data File | Microsoft Visual Studio | Microsoft Corporation |
| .JTF | Various Developers | Raster Image Files | JPEG Tagged Interchange Format | Microsoft Windows Photos | Microsoft Corporation |
| .JTP | Microsoft Corporation | Page Layout Files | Windows Journal Template | Microsoft Windows Journal | Microsoft Corporation |
| .JTX | Microsoft Corporation | Page Layout Files | XPS Document File | Microsoft XPS Viewer | Microsoft Corporation |
| .JVS | Opera Software | Web Files | JavaScript Proxy Autoconfig File | Internet Explorer | Microsoft Corporation |
| .JW | Microsoft Corporation | Data Files | Microsoft WordPad Document | Microsoft WordPad | Microsoft Corporation |
| .JWK | Douglas Crockford | Encrypted Files | JSON Web Key | Microsoft Notepad | Microsoft Corporation |
| .JWL | Roxio | Raster Image Files | Roxio Jewel Case File | JustWrite Office | Microsoft Corporation |
| .JXR | Microsoft Corporation | Raster Image Files | JPEG XR Image | Microsoft Windows Photos | Microsoft Corporation |
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