| .WAS | Symantec Corporation | File sviluppatore | Symantec Procomm Plus Script | Procomm Plus | Symantec Corporation |
| .WBF | Microsoft Corporation | File sviluppatore | Microsoft Windows Batch File | Microsoft Notepad | Microsoft Corporation |
| .WDG | PC SOFT | File sviluppatore | WinDev Procedures | WinDev | PC SOFT |
| .WDGT | Opera Software | File sviluppatore | Opera Widget File | Opera | Opera Software |
| .WDL | Conitec | File sviluppatore | World Definition Language Script | 3D GameStudio | Conitec |
| .WDO | PC SOFT | File sviluppatore | WinDev Component List | WinDev | PC SOFT |
| .WDP | Microsoft Corporation | File sviluppatore | Windows Media Photo File | Adobe Photoshop | Adobe Systems Incorporated |
| .WDW | PC SOFT | File sviluppatore | WinDev Window File | WinDev | PC SOFT |
| .WFS | Microsoft Corporation | File sviluppatore | Microsoft Windows Installation Script | Microsoft Windows | Microsoft Corporation |
| .WHS | X-Ways | File sviluppatore | WinHex Script | WinHex | X-Ways |
| .WID | SAP | File sviluppatore | SAP BusinessObjects Web Intelligence File | SAP BusinessObjects Business Intelligence | SAP |
| .WOD | Apple | File sviluppatore | WebObjects Declaration File | WebObjects | Apple |
| .WORKSHEET | Bare Bones Software | File sviluppatore | BBEdit Shell Worksheet File | BBEdit | Bare Bones Software |
| .WOWPROJ | Open Source | File sviluppatore | AddOn Studio Project File | Addon Studio for World of Warcraft | Open Source |
| .WPW | PC SOFT | File sviluppatore | WinDev Mobile Window File | WinDev | PC SOFT |
| .WSC | Open Source | File sviluppatore | WonderSwan Color Game ROM | Oswan | Open Source |
| .WSP | Microsoft Corporation | File sviluppatore | SharePoint Solution Package | Microsoft Visual Studio | Microsoft Corporation |
| .WXS | Outercurve Foundation | File sviluppatore | WiX Source File | WiX Toolset | .NET Foundation |
| .X | Microsoft Corporation | File sviluppatore | DirectX Model File | Microsoft DirectX SDK | Microsoft Corporation |
| .XAML | Microsoft Corporation | File sviluppatore | Extensible Application Markup Language | Microsoft Notepad | Microsoft Corporation |
| .XAP | Microsoft Corporation | File sviluppatore | XACT Project | Microsoft XACT | Microsoft Corporation |
| .XBD | Xerox | File sviluppatore | DocuWorks Binder File | DocuWorks Viewer Light | Fuji Xerox |
| .XBF | Microsoft Corporation | File sviluppatore | XAML Binary File | Microsoft Visual Studio | Microsoft Corporation |
| .XBL | Open Source | File sviluppatore | XBLite Source Code File | XBLite | Open Source |
| .XBS | XnSoft | File sviluppatore | XnView Script | XnConvert | XnSoft |
| .XCAPPDATA | Apple | File sviluppatore | Xcode App Data File | Apple Xcode | Apple |
| .XCARCHIVE | Apple | File sviluppatore | Xcode Archive | Apple Xcode | Apple |
| .XCCRASHPOINT | Apple | File sviluppatore | Xcode Crash Point File | Apple Xcode | Apple |
| .XCL | Jeffrey C. Johnson | File sviluppatore | XTree For Windows Script | XTreeGold | Jeffrey C. Johnson |
| .XCMD | Bill Atkinson | File sviluppatore | HyperCard External Commands | HyperCard | Bill Atkinson |
| .XCODE | Apple | File sviluppatore | Xcode Project | Apple Xcode | Apple |
| .XCODEPROJ | Apple | File sviluppatore | Xcode Project | Apple Xcode | Apple |
| .XCSCHEME | Apple | File sviluppatore | Xcode Scheme File | Apple Xcode | Apple |
| .XCWORKSPACE | Apple | File sviluppatore | Xcode Workspace File | Apple Xcode | Apple |
| .XIB | Apple | File sviluppatore | Interface Builder File | Apple Interface Builder | Apple |
| .XIN | Stilo | File sviluppatore | OmniMark Include Data | OmniMark | Stilo |
| .XL | Microsoft Corporation | File sviluppatore | Excel Spreadsheet | Microsoft Excel | Microsoft Corporation |
| .XPB | Tony D. Jones | File sviluppatore | XPB! Basic Source Code | XPB! Basic | Tony D. Jones |
| .XPP | Microsoft Corporation | File sviluppatore | X++ Source Code File | Microsoft Dynamics AX | Microsoft Corporation |
| .XQL | Open Source | File sviluppatore | XML Query Language File | Microsoft Notepad | Microsoft Corporation |
| .XR | Red Hat | File sviluppatore | Cygwin Ld-scripts Data | Cygwin | Open Source |
| .XSC | Microsoft Corporation | File sviluppatore | Visual Studio Dataset Internal Info File | Microsoft Visual Studio | Microsoft Corporation |
| .XSD | Unknown Developer | File sviluppatore | XML Schema Definition | Microsoft Visual Studio | Microsoft Corporation |
| .XT | Derick Rethans | File sviluppatore | Xdebug Trace File | Xdebug | Derick Rethans |
| .XVP | Sun Microsystems | File sviluppatore | Xview Package File | XView | Sun Microsystems |
| .Y | Microsoft Corporation | File sviluppatore | Subsampled Raw YUV Image | Microsoft Windows Media Player | Microsoft Corporation |
| .YAB | Open Source | File sviluppatore | Yabasic Source Code | Yabasic | Open Source |
| .YAML | Unknown Developer | File sviluppatore | YAML Document | Microsoft Notepad | Microsoft Corporation |
| .YML | Unknown Developer | File sviluppatore | YAML Document | Microsoft Notepad | Microsoft Corporation |
| .YMP | Open Source | File sviluppatore | YaST Metapackage File | YaST | Open Source |
| .ZARGO | Open Source | File sviluppatore | ArgoUML Model | ArgoUML | Open Source |
| .ZFD | Unknown Developer | File sviluppatore | ABC Zeroadic Functions Data | ABC Programming langauge | Open Source |
| .ZMS | Check Point Software Technologies Ltd. | File sviluppatore | ZoneAlarm Mailsafe Renamed Data | WinZip | Corel |
| .ZPD | Open Source | File sviluppatore | ABC Zeroadic Predicates | ABC Programming langauge | Open Source |
| .ZPL | Inmatrix Media Solutions | File sviluppatore | Zoom Player Play List | Zoom Player | Inmatrix Media Solutions |
| .ZSH | AT&T Labs | File sviluppatore | Unix Shell Script | Unix | AT&T Labs |
| .ZTS | David P. Alverson | File sviluppatore | ZTerm Script | ZTerm | David P. Alverson |
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