| .X | Microsoft Corporation | Arquivos de desenvolvedor | DirectX Model File | Microsoft DirectX SDK | Microsoft Corporation |
| .XAML | Microsoft Corporation | Arquivos de desenvolvedor | Extensible Application Markup Language | Microsoft Notepad | Microsoft Corporation |
| .XAP | Microsoft Corporation | Arquivos de desenvolvedor | XACT Project | Microsoft XACT | Microsoft Corporation |
| .XBD | Xerox | Arquivos de desenvolvedor | DocuWorks Binder File | DocuWorks Viewer Light | Fuji Xerox |
| .XBF | Microsoft Corporation | Arquivos de desenvolvedor | XAML Binary File | Microsoft Visual Studio | Microsoft Corporation |
| .XBL | Open Source | Arquivos de desenvolvedor | XBLite Source Code File | XBLite | Open Source |
| .XBS | XnSoft | Arquivos de desenvolvedor | XnView Script | XnConvert | XnSoft |
| .XCAPPDATA | Apple | Arquivos de desenvolvedor | Xcode App Data File | Apple Xcode | Apple |
| .XCARCHIVE | Apple | Arquivos de desenvolvedor | Xcode Archive | Apple Xcode | Apple |
| .XCCRASHPOINT | Apple | Arquivos de desenvolvedor | Xcode Crash Point File | Apple Xcode | Apple |
| .XCL | Jeffrey C. Johnson | Arquivos de desenvolvedor | XTree For Windows Script | XTreeGold | Jeffrey C. Johnson |
| .XCMD | Bill Atkinson | Arquivos de desenvolvedor | HyperCard External Commands | HyperCard | Bill Atkinson |
| .XCODE | Apple | Arquivos de desenvolvedor | Xcode Project | Apple Xcode | Apple |
| .XCODEPROJ | Apple | Arquivos de desenvolvedor | Xcode Project | Apple Xcode | Apple |
| .XCSCHEME | Apple | Arquivos de desenvolvedor | Xcode Scheme File | Apple Xcode | Apple |
| .XCWORKSPACE | Apple | Arquivos de desenvolvedor | Xcode Workspace File | Apple Xcode | Apple |
| .XIB | Apple | Arquivos de desenvolvedor | Interface Builder File | Apple Interface Builder | Apple |
| .XIN | Stilo | Arquivos de desenvolvedor | OmniMark Include Data | OmniMark | Stilo |
| .XL | Microsoft Corporation | Arquivos de desenvolvedor | Excel Spreadsheet | Microsoft Excel | Microsoft Corporation |
| .XPB | Tony D. Jones | Arquivos de desenvolvedor | XPB! Basic Source Code | XPB! Basic | Tony D. Jones |
| .XPP | Microsoft Corporation | Arquivos de desenvolvedor | X++ Source Code File | Microsoft Dynamics AX | Microsoft Corporation |
| .XQL | Open Source | Arquivos de desenvolvedor | XML Query Language File | Microsoft Notepad | Microsoft Corporation |
| .XR | Red Hat | Arquivos de desenvolvedor | Cygwin Ld-scripts Data | Cygwin | Open Source |
| .XSC | Microsoft Corporation | Arquivos de desenvolvedor | Visual Studio Dataset Internal Info File | Microsoft Visual Studio | Microsoft Corporation |
| .XSD | Unknown Developer | Arquivos de desenvolvedor | XML Schema Definition | Microsoft Visual Studio | Microsoft Corporation |
| .XT | Derick Rethans | Arquivos de desenvolvedor | Xdebug Trace File | Xdebug | Derick Rethans |
| .XVP | Sun Microsystems | Arquivos de desenvolvedor | Xview Package File | XView | Sun Microsystems |
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