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