| .TXK | Unknown Developer | Ficheiros de Texto | Karaoke Song Lyrics | KaraDOS | Unknown Developer |
| .TXM | CFP Software Limited | Ficheiros de Dados | CFPwinMan Document File | Unknown Software | Unknown Developer |
| .TXW | Yamaha Corporation | Ficheiros de Áudio | Yamaha TX16W Audio File | TX16WConverter | Unknown Developer |
| .TY2 | Unknown Developer | Uncommon Files | Gualtieri Theme Maker File | Gualtieri Theme Maker | Unknown Developer |
| .TYP | Unknown Developer | Uncommon Files | Thumbs32 File | Unknown Software | Unknown Developer |
| .TZG | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .U2C | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .U2E | Unreal Software | Uncommon Files | Unreal Game Translation | Unknown Software | Unknown Developer |
| .U4A | Unknown Developer | Ficheiros Executáveis | UnInstaller Archive File | Unknown Software | Unknown Developer |
| .U8 | Nintendo | Ficheiros de Jogo | Mario Kart Wii Archive File | Unknown Software | Unknown Developer |
| .UAD | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .UAP | Siemens AG | Ficheiros Compactados | SIMATIC WinCC User Archive Export | Unknown Software | Unknown Developer |
| .UB1 | Unknown Developer | Ficheiros de Vídeo | X-Driven DRS-1100 Recorded Video | Unknown Software | Unknown Developer |
| .UBF | Marker Communication | Ficheiros de Dados | MXT3010 Universal Boot Format | Unknown Software | Unknown Developer |
| .UBI | Ubisoft | Ficheiros de Jogo | Ubisoft Logo Animation | Unknown Software | Unknown Developer |
| .UBZ | Unknown Developer | Ficheiros de Dados | Open-Sankore Document | Open-Sankoré | Unknown Developer |
| .UC2 | Nico De Vries | Ficheiros Compactados | UltraCompressor 2 Archive | AIPNL UltraCompressor 2.3 or UltraExpander | Unknown Developer |
| .UCA | Unknown Developer | Uncommon Files | Unicode Collation Algorithm Data File | Unknown Software | Unknown Developer |
| .UCI | SysDevSoftware | Ficheiros de Cópia de Segurança | UFS Explorer Backup File | Unknown Software | Unknown Developer |
| .UK_ | Unknown Developer | Uncommon Files | Mcguiness File | Unknown Software | Unknown Developer |
| .UND | DHI | Ficheiros de Dados | MOUSE Urban Network Datafile | Unknown Software | Unknown Developer |
| .UNIX BAR COMPRESSED FILE ARCHIVE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .UNKNOWN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .UNM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .UP | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .UPD | Various Developers | Ficheiros de Texto | Program Update Information | Unknown Software | Unknown Developer |
| .UPDATED | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .UTB | Unknown Developer | Ficheiros de Cópia de Segurança | µTBackup Backup | µTBackup | Unknown Developer |
| .UVC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .UVZ | Unknown Developer | Ficheiros Compactados | DECE ZIP Compressed Data | Unknown Software | Unknown Developer |
| .UXR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .V2Q | Texas Instruments, Inc. | Ficheiros de Dados | TI Voyage 200 Certificate File | Unknown Software | Unknown Developer |
| .V48 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .V50 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .VAD | Blue Marble Geographics | Ficheiros de Dados | Mapper Variant | Unknown Software | Unknown Developer |
| .VAR | M:Pact Technologies | Ficheiros de Desenvolvedor | Variable Data | Icon Author | Unknown Developer |
| .VAX | Unknown Developer | Uncommon Files | Pbm Plus File | Unknown Software | Unknown Developer |
| .VCMP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VCY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VC_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VDH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VDMPVF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VDO | Unknown Developer | Ficheiros Executáveis | Heathen Virus File | Unknown Software | Unknown Developer |
| .VEX | Unknown Developer | Ficheiros de Dados | VLBI Experiment Definition Data | Very Long Baseline Interferometry (VLBI) | Unknown Developer |
| .VFF | Sun Microsystems | Ficheiros de Imagem Raster | Sun TAAC Graphic File | Unknown Software | Unknown Developer |
| .VHA | Macromedia | Uncommon Files | Macromedia Sitespring File | Unknown Software | Unknown Developer |
| .VHDPMEM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .VHP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VIEWER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .VI_ | Unknown Developer | Ficheiros de Áudio | Samplitude Compressed File | Audio Waveprg | Unknown Developer |
| .VLQ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VMCZ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .VOICE | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .VOT | Unknown Developer | Ficheiros Raw de Câmera | Recorded Video File | Unknown Software | Unknown Developer |
| .VPB | Unknown Developer | Ficheiros de Imagem Raster | Quantel VPB Image Format | Unknown Software | Unknown Developer |
| .VQE | Yamaha Corporation | Ficheiros de Áudio | Yamaha Sound-VQ Locator File | Unknown Software | Unknown Developer |
| .VQL | Yamaha Corporation | Ficheiros de Áudio | Yamaha Sound-VQ Locator File | Unknown Software | Unknown Developer |
| .VRA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VSR | Microsoft Corporation | Ficheiros de Base de Dados | Microsoft Access Report | Unknown Software | Unknown Developer |
| .VT | Unknown Developer | Ficheiros de Dados | VT100 Terminal Data File | Unknown Software | Unknown Developer |
| .VUX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .VVR | Unknown Developer | Ficheiros de Dados | ViVerify Repair File | ViVerify Repair | Unknown Developer |
| .VXX | Unknown Developer | Ficheiros de Sistema | IOsubsys | Unknown Software | Unknown Developer |
| .VYB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .W2F | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .W73 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .WC4 | Unknown Developer | Uncommon Files | WinCheckIt4 Settings File | Unknown Software | Unknown Developer |
| .WEBARCHIV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .WEI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .WF2 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .WINDOWSLIV | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .WINDOWSMED | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .WLF | Microsoft Corporation | Ficheiros de Dados | Microsoft Windows 98/Me Dr. Watson Log | Unknown Software | Unknown Developer |
| .WLO | JinKe Electronic Company | Ficheiros de eBook | WOLF eBook | Unknown Software | Unknown Developer |
| .WLT | Unknown Developer | Ficheiros Gráficos | WaveL Wavelet Compressed Graphic | Unknown Software | Unknown Developer |
| .WM5 | Unknown Developer | Uncommon Files | Dyndlg2 File | Dyndlg2 | Unknown Developer |
| .WMH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .WMO | World Meteorological Organization | Ficheiros de Dados | WMO Binary Format File | Unknown Software | Unknown Developer |
| .WSQ | UTAU Team | Ficheiros de Áudio | UTAU File | Unknown Software | Unknown Developer |
| .WS_ | Unknown Developer | Ficheiros de Configurações | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .WVT | Unknown Developer | Ficheiros de Dados | Compuwx Export Data | Unknown Software | Unknown Developer |
| .WWZ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .WXW | WiNRADiO | Uncommon Files | WiNRADiO Extended Wave File | Unknown Software | Unknown Developer |
| .X23 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .X2M | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .X33 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .X3M | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .XACT | XactSystems | Uncommon Files | XactScript File | Unknown Software | Unknown Developer |
| .XAD | Riven the Mage | Ficheiros de Áudio | eXotic ADlib Sound Format File | Unknown Software | Unknown Developer |
| .XAK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .XB~ | Unknown Developer | Uncommon Files | IC File | Unknown Software | Unknown Developer |
| .XCK | Unknown Developer | Ficheiros de Dados | Extended Crack File | Unknown Software | Unknown Developer |
| .XD | Unknown Developer | Ficheiros de Dados | CCS-lib.2 Examples Motif-CC File | Unknown Software | Unknown Developer |
| .XDE | Unknown Developer | Ficheiros de Imagem 3D | Tgif-3.0 File | Unknown Software | Unknown Developer |
| .XDSL | D-Link Corporation | Ficheiros de Sistema | D-link Router UK Firmware Update File | Unknown Software | Unknown Developer |
| .XED | Unknown Developer | Ficheiros de Dados | XED Chemical Modeler Input/Output Data | Unknown Software | Unknown Developer |
| .XI1 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .XLTD | Unknown Developer | Uncommon Files | Thunder Temporary Data File | Unknown Software | Unknown Developer |
| .XMAS | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .XMIT | IBM | Ficheiros de Dados | IBM XMIT Data | Unknown Software | Unknown Developer |
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