| .TEMPLATE | Apple | Ficheiros de Texto | Apple Pages Template | Apple Pages | Apple |
| .TEX | Open Source | Ficheiros de Texto | LaTeX Source Document | gVim | Open Source |
| .TEXT | Sublime HQ Pty Ltd | Ficheiros de Texto | Plain Text File | Microsoft Notepad | Microsoft Corporation |
| .TEXTCLIPPING | Apple | Ficheiros de Texto | Mac OS X Text Clipping File | Apple Finder | Apple |
| .TGP | Planetside Software | Ficheiros de Texto | Terragen Plugin | Planetside Software Terragen | Planetside Software |
| .THP | thakis | Ficheiros de Texto | Wii/GameCube Video File | thpplay | thakis |
| .THR | PTC | Ficheiros de Texto | Pro/ENGINEER File | Pro/ENGINEER | PTC |
| .TLB | Various Developers | Ficheiros de Texto | VAX Text Library | Apple TextEdit | Apple |
| .TLD | The Eclipse Foundation | Ficheiros de Texto | Tag Library Descriptor File | Notepad++ | Don Ho |
| .TLSET | BINProjects | Ficheiros de Texto | Customization (Settings) File | Text Locker | BINProjects |
| .TLW | TLWallaceNET Publishing Inc. | Ficheiros de Texto | TLWallace.NET Document | TLWallace.NET | TLWallaceNET Publishing Inc. |
| .TMD | SoftMaker Software | Ficheiros de Texto | TextMaker Document | Softmaker Office | SoftMaker Software |
| .TMV | SoftMaker Software | Ficheiros de Texto | TextMaker Template | Softmaker Office | SoftMaker Software |
| .TODO | Open Source | Ficheiros de Texto | Simple Project ToDo Database | Simple Project ToDo | Open Source |
| .TPH | Borland | Ficheiros de Texto | Turbo Pascal Help File | Turbo Pascal | Embarcadero Technologies |
| .TPL | Unknown Developer | Ficheiros de Texto | Document Template | NoteTab | Fookes Software |
| .TTE | Microsoft Corporation | Ficheiros de Texto | Private Character Editor File | Microsoft Private Character Editor | Microsoft Corporation |
| .TTF | Apple | Ficheiros de Texto | TrueType Font | CorelDRAW Graphics Suite | Corel |
| .TTW | IBM | Ficheiros de Texto | IBM Rational Tau Workspace | Rational Tau | IBM |
| .TUN | High Voltage Software | Ficheiros de Texto | LEGO Racers Audio File | Audacity | Open Source |
| .TWD | SimTech | Ficheiros de Texto | MindMapper Brainstorm And Visual Map | MindMapper | SimTech |
| .TX8 | Microsoft Corporation | Ficheiros de Texto | DOS Text File | Notepad++ | Don Ho |
| .TXA | Sentel Advance | Ficheiros de Texto | Midas ViewPoint Text Array List | Sentel Advance Midas ViewPoint | Sentel Advance |
| .TXD | DK22Pac | Ficheiros de Texto | Game Texture Dictionary | Magic.TXD | DK22Pac |
| .TXE | Open Source | Ficheiros de Texto | Enriched Text File | Microsoft Notepad | Microsoft Corporation |
| .TXK | Unknown Developer | Ficheiros de Texto | Karaoke Song Lyrics | KaraDOS | Unknown Developer |
| .TXT | Sublime HQ Pty Ltd | Ficheiros de Texto | Plain Text File | Microsoft Notepad | Microsoft Corporation |
| .U3I | Interscape | Ficheiros de Texto | U3 Application Information File | U3 Launcher | Interscape |
| .UDF | Open Source | Ficheiros de Texto | Universal Disk Format Image File | 7-Zip | Igor Pavlov |
| .UNX | Unknown Developer | Ficheiros de Texto | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .UOF | The Document Foundation | Ficheiros de Texto | Uniform Office Document | Apache OpenOffice | Apache Software Foundation |
| .UPD | Various Developers | Ficheiros de Texto | Program Update Information | Microsoft Notepad | Microsoft Corporation |
| .UPDF | Software995 | Ficheiros de Texto | UltraPdf Publication File | UltraPdf | Software995 |
| .UPM | Livescribe | Ficheiros de Texto | PenMail Document | PenMail | Livescribe |
| .USE | Unknown Developer | Ficheiros de Texto | Log File | Microsoft Notepad | Microsoft Corporation |
| .UTXT | Open Source | Ficheiros de Texto | Unicode Text File | Microsoft Notepad | Microsoft Corporation |
| .VAD | Blue Marble Geographics | Ficheiros de Texto | Mapper Variant | Global Mapper | Blue Marble Geographics |
| .VCG | Microsoft Corporation | Ficheiros de Texto | SharePoint WorkSpace vCard Contact | Microsoft Office | Microsoft Corporation |
| .VCT | Clark Labs | Ficheiros de Texto | IDRISI Vector Image | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .VET | Lectra | Ficheiros de Texto | Lectra Cutter's Must File | Modaris | Lectra |
| .VFK | Sublime HQ Pty Ltd | Ficheiros de Texto | Czech Cadastral Exchange Data Format | Notepad++ | Don Ho |
| .VIS | Microsoft Corporation | Ficheiros de Texto | Visibility Image File | Microsoft Flight Simulator 2004 | Microsoft Corporation |
| .VM | Apache HTTP Project Management Committee | Ficheiros de Texto | Velocity Template | Apache Velocity | Open Source |
| .VMM | Microsoft Corporation | Ficheiros de Texto | Microsoft 3D Movie Maker Video File | 3D Movie Maker | Microsoft Corporation |
| .VNA | JVCKenwood Group | Ficheiros de Texto | JVC JLIP Image | JVC Multimedia Navigator | JVCKenwood Group |
| .VNT | Sony | Ficheiros de Texto | Sony Ericsson Mobile Phone vNote File | Microsoft Notepad | Microsoft Corporation |
| .VTS | Intel Corporation | Ficheiros de Texto | Intel VTune Program Performance Monitor Project | Intel VTune | Intel Corporation |
| .VW | Lifetree Software | Ficheiros de Texto | Volkswriter Text File | Corel WordPerfect X8 | Corel |
| .W61 | Corel Corporation | Ficheiros de Texto | Corel WordPerfect 6.1 Document | Corel WordPerfect | Corel |
| .WBK | Microsoft Corporation | Ficheiros de Texto | Word Document Backup | Microsoft Word | Microsoft Corporation |
| .WCD | Microsoft Corporation | Ficheiros de Texto | Microsoft Works Calendar File | Microsoft Works | Microsoft Corporation |
| .WCN | Anthemion Software Ltd. | Ficheiros de Texto | Writer's Café Notebook Data | Writer's Café | Anthemion Software Ltd. |
| .WCP | Corel | Ficheiros de Texto | WinDVD Creator Project File | Corel WinDVD | Corel |
| .WD2 | Rockstar Games | Ficheiros de Texto | American Idol Data | American Idol | Rockstar Games |
| .WIS | Flexera Software | Ficheiros de Texto | InstallShield Install Help File | InstallShield | Flexera Software |
| .WKB | Corel | Ficheiros de Texto | Corel WordPerfect Document | Corel WordPerfect | Corel |
| .WKF | KeyFocus | Ficheiros de Texto | KF Web Server Default Control File | KF Web Server | KeyFocus |
| .WN | WN | Ficheiros de Texto | WN Web File | WN | Open Source |
| .WOL | Don Johnston, Inc. | Ficheiros de Texto | Write:OutLoud File | Write: OutLoud | Don Johnston, Inc. |
| .WP | Corel | Ficheiros de Texto | WordPerfect Document | Corel WordPerfect X8 | Corel |
| .WP4 | Corel | Ficheiros de Texto | WordPerfect 4 Document | Corel WordPerfect X8 | Corel |
| .WP5 | Corel | Ficheiros de Texto | WordPerfect 5 Document | Corel WordPerfect X8 | Corel |
| .WP6 | Corel | Ficheiros de Texto | WordPerfect 6 Document | Corel WordPerfect X8 | Corel |
| .WPA | Swiftpage, Inc. | Ficheiros de Texto | ACT Word Processing Document | Swiftpage Act | Swiftpage, Inc. |
| .WPD | Corel | Ficheiros de Texto | WordPerfect Document | Corel WordPerfect X8 | Corel |
| .WPH | Phoenix Technologies | Ficheiros de Texto | Phoenix BIOS File | Phlash16.exe | Phoenix Technologies |
| .WPL | Microsoft Corporation | Ficheiros de Texto | Windows Media Player Playlist | Microsoft Windows Media Player | Microsoft Corporation |
| .WPS | Kingsoft | Ficheiros de Texto | Kingsoft Writer Document | WPS Office | Kingsoft |
| .WPT | Corel | Ficheiros de Texto | WordPerfect Template | Corel WordPerfect X8 | Corel |
| .WPW | PC SOFT | Ficheiros de Texto | WinDev Mobile Window File | WinDev | PC SOFT |
| .WRD | Open Source | Ficheiros de Texto | COREX Manually Verified Word Segmentation Data | COREX | Open Source |
| .WRF | Cisco Systems | Ficheiros de Texto | WebEx Recording | Cisco WebEx Player | Cisco Systems |
| .WRI | Microsoft Corporation | Ficheiros de Texto | Microsoft Write Document | LibreOffice | The Document Foundation |
| .WRT | IBM | Ficheiros de Texto | IBM Writing Assistant Document | MS-DOS | Microsoft Corporation |
| .WS | MicroPro International Corp. | Ficheiros de Texto | WordStar Document | Corel WordPerfect X8 | Corel |
| .WS2 | MicroPro International Corp. | Ficheiros de Texto | WordStar Document | Corel WordPerfect X8 | Corel |
| .WSA | Sonicle | Ficheiros de Texto | WebSynth Media File | WebSynth | Sonicle |
| .WSD | MicroPro International Corp. | Ficheiros de Texto | WordStar Document | Corel WordPerfect X8 | Corel |
| .WSR | Genius Media Group | Ficheiros de Texto | Crossword Genius Data | Crossword Genius | Genius Media Group |
| .WT2 | MicroVision Development, Inc. | Ficheiros de Texto | WordExpress Document Template | WordExpress | MicroVision Development, Inc. |
| .WTT | HamsterCoders | Ficheiros de Texto | Write! Document | Write! | HamsterCoders |
| .WTX | Stefan Hagel | Ficheiros de Texto | Text Document | Microsoft Notepad | Microsoft Corporation |
| .XAMLX | Microsoft Corporation | Ficheiros de Texto | Microsoft Visual Studio XAMLX Document | Microsoft Visual Studio | Microsoft Corporation |
| .XC | Open Source | Ficheiros de Texto | Plain Text Source-code File | Notepad++ | Don Ho |
| .XCP | Advanced Systems Concepts, Inc. | Ficheiros de Texto | XLNT Script Command | XLNT | Advanced Systems Concepts, Inc. |
| .XMS | Alexander Gross | Ficheiros de Texto | Music Studio Project | Alexander Gross Music Studio | Alexander Gross |
| .XNG | Unknown Developer | Ficheiros de Texto | FishXing Culvert Data File | FishXing | Unknown Developer |
| .XOML | Microsoft Corporation | Ficheiros de Texto | Microsoft Visual Studio XOML File | Microsoft Visual Studio | Microsoft Corporation |
| .XPROJ | Microsoft Corporation | Ficheiros de Texto | Microsoft Visual Studio ASP.NET Project | Microsoft Visual Studio | Microsoft Corporation |
| .XSR | Bentley Systems | Ficheiros de Texto | Bentley GEOPAK XS List Report | GEOPAK Civil Engineering Suite | Bentley Systems |
| .XTG | Ben Stöver | Ficheiros de Texto | Extensible TreeGraph File | TreeGraph | Ben Stöver |
| .XTX | Flambeaux Software | Ficheiros de Texto | xText File | xText | Flambeaux Software |
| .XVG | Open Source | Ficheiros de Texto | Grace Document | GRACE | Open Source |
| .XWP | Xerox | Ficheiros de Texto | Xerox Writer Text Document | Xerox Writer | Fuji Xerox |
| .XXN | Dimenzo, Inc. | Ficheiros de Texto | Dimenzo Text Editor Document | Dimenzo Text Editor | Dimenzo, Inc. |
| .XY | XyQuest | Ficheiros de Texto | XYWrite Document | Corel WordPerfect | Corel |
| .XY3 | David Erickson | Ficheiros de Texto | XYWrite III Document | XYWrite for Windows | David Erickson |
| .XY4V | XyQuest | Ficheiros de Texto | XYWrite IV Document File | XyWrite | XyQuest |
| .XYP | The Technology Group | Ficheiros de Texto | XYWrite Plus Document | XYWrite Plus | The Technology Group |
| .XYW | David Erickson | Ficheiros de Texto | XYWrite For Windows Document | XYWrite for Windows | David Erickson |
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