| .TSD | Software AG | Gegevensbestanden | Tamino Schema Editor File | Tamino Schema Editor | Software AG |
| .TSG | CSIRO | Gegevensbestanden | TSG Data File | TSG | CSIRO |
| .TSJ | Topcon Totalcare | Gegevensbestanden | TopCon TopSurf Job | TopSURV | Topcon Totalcare |
| .TSK | Microsoft Corporation | Gegevensbestanden | Pocket PC Skin | Microsoft ActiveSync | Microsoft Corporation |
| .TSL | Subsite Electronics | Gegevensbestanden | Tracker Status Log | TSR Mobile | Subsite Electronics |
| .TSN | National Center for Biotechnology Information | Gegevensbestanden | RatLab Toolkit Data | RatLab | National Center for Biotechnology Information |
| .TSS | Galactix Software | Gegevensbestanden | Team Sports Scheduling System Project | Team Sports Scheduling System | Galactix Software |
| .TST | Measurement Computing | Gegevensbestanden | TestPoint Test File | Measurement Computing TestPoint | Measurement Computing |
| .TSV | Unknown Developer | Gegevensbestanden | Tab Separated Values File | Microsoft Excel | Microsoft Corporation |
| .TSX | Galactix Software | Gegevensbestanden | Team Sports Scheduling System XML Project | Team Sports Scheduling System | Galactix Software |
| .TT16 | Intuit, Inc. | Gegevensbestanden | TurboTax 2016 Tax Return | Intuit TurboTax | Intuit, Inc. |
| .TT17 | Intuit, Inc. | Gegevensbestanden | TurboTax 2017 Tax Return | Intuit TurboTax | Intuit, Inc. |
| .TTD | SensiTech, Inc. | Gegevensbestanden | TempTale Data File | TempTale Manager Desktop | SensiTech, Inc. |
| .TTI | Delphi Softwares | Gegevensbestanden | Trojan File | Beast (Trojan Horse) | Delphi Softwares |
| .TTK | Alchemy Software | Gegevensbestanden | Catalyst Translation Toolkit | Alchemy Catalyst | Alchemy Software |
| .TTN | Scotweb Marketing Ltd. | Gegevensbestanden | Tartan Generator Saved Tartan | Tartan Designer | Scotweb Marketing Ltd. |
| .TTS | SumTotal Systems | Gegevensbestanden | ToolBook Translation System File | ToolBook | SumTotal Systems |
| .TTT | Interscape | Gegevensbestanden | TestTrack Pro Database File | TestTrack Pro | Interscape |
| .TTV | SensiTech, Inc. | Gegevensbestanden | TempTale Manager Desktop Data | TempTale Manager Desktop | SensiTech, Inc. |
| .TTX | Simple DirectMedia Layer (SDL) | Gegevensbestanden | Trados TagEditor File | SDL Trados | SDL Trados |
| .TTY | Adobe Systems Incorporated | Gegevensbestanden | Adobe Dreamweaver Data | Adobe Dreamweaver | Adobe Systems Incorporated |
| .TV1 | Corel | Gegevensbestanden | Corel WordPerfect Data | Corel WordPerfect | Corel |
| .TV5 | Corel Corporation | Gegevensbestanden | Corel WordPerfect Overflow File | Corel WordPerfect X8 | Corel |
| .TV6 | Corel Corporation | Gegevensbestanden | Corel WordPerfect Overflow Data File | Corel WordPerfect X8 | Corel |
| .TVC | IMSI/Design, LLC | Gegevensbestanden | Turbo View & Convert Batch Presets File | IMSI TurboCAD Deluxe | Redway3d and others |
| .TVL | Intuit, Inc. | Gegevensbestanden | TurboTax File | Intuit TurboTax | Intuit, Inc. |
| .TVR | PremiumSoft CyberTech Ltd | Gegevensbestanden | Navicat For MySQL Data | Navicat for MySQL | PremiumSoft CyberTech Ltd |
| .TWB | Tableau Software | Gegevensbestanden | Tableau Workbook File | Tableau Desktop | Tableau Software |
| .TWBX | Tableau Software | Gegevensbestanden | Tableau Packaged Workbook | Tableau Desktop | Tableau Software |
| .TWG | SINTEF | Gegevensbestanden | CARE-S InfoWorks Trade Waste Data | CARE-S | SINTEF |
| .TWK | FileMaker Inc. | Gegevensbestanden | T-Works Manager Data File | T-Works Manager | FileMaker Inc. |
| .TWM | Open Source | Gegevensbestanden | TheWord Module | theWord | Costas stergiou |
| .TWO | Apple | Gegevensbestanden | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .TWP | Mark O. Hill & Petr Smilauer | Gegevensbestanden | WinTWINS Project File | TWINSPAN | Mark O. Hill & Petr Smilauer |
| .TX | Computerinsel GmbH | Gegevensbestanden | Photoline 5 Help Data | Photoline | Computerinsel GmbH |
| .TXD | DK22Pac | Gegevensbestanden | Game Texture Dictionary | Magic.TXD | DK22Pac |
| .TXM | CFP Software Limited | Gegevensbestanden | CFPwinMan Document File | CFPwinMan | CFP Software Limited |
| .TXN | MySpace | Gegevensbestanden | MySpace IM Log | MySpaceIM | MySpace |
| .TXTDT | DVD Shrink | Gegevensbestanden | TitleWriter DVD Text Data File | TitleWriter | DVD Shrink |
| .TXX | National Library of Medicine | Gegevensbestanden | Insight Segmentation And Registration Toolkit Data File | Insight Segmentation and Registration Toolkit | Open Source |
| .U98 | Sierra Entertainment | Gegevensbestanden | USA 98 Streets & Destinations Tour Data File | USA 98 Streets & Destinations | Sierra Entertainment |
| .UBF | Marker Communication | Gegevensbestanden | MXT3010 Universal Boot Format | Unknown Software | Unknown Developer |
| .UBJ | The Buzz Media | Gegevensbestanden | Universal Binary JSON File | Universal Binary JSON Binary Java Library | GitHub, Inc. |
| .UBK | Craftsman Book Company | Gegevensbestanden | National Construction Estimator 32 User Costbook File | National Construction Estimator | Craftsman Book Company |
| .UBOX | Giant Army | Gegevensbestanden | Universe Sandbox Saved Data | Universe Sandbox | Giant Army |
| .UBZ | Unknown Developer | Gegevensbestanden | Open-Sankore Document | Open-Sankoré | Unknown Developer |
| .UCC | National LawForms, Inc. | Gegevensbestanden | Uniform Commercial Code Form File | National UCC Financing Statement Software | National LawForms, Inc. |
| .UCCAPILOG | Microsoft Corporation | Gegevensbestanden | Microsoft UCC API Log File | Microsoft Windows Live Messenger | Microsoft Corporation |
| .UCF | Cisco Systems | Gegevensbestanden | WebEx Media File | WebEx Meetings | Cisco Systems |
| .UCV | Anthony R. Daly | Gegevensbestanden | Universal Converter Conversion File | Universal Converter | Soft Solutions Ltd. |
| .UCX | IBM | Gegevensbestanden | Rational XDE Data | Rational Rose | IBM |
| .UD | Nuance | Gegevensbestanden | OmniPage User Dictionary | Nuance OmniPage Pro X | Nuance |
| .UDT | wyDay | Gegevensbestanden | User Defined Type Data | WyBuild | wyDay |
| .UDX | Power Integrations | Gegevensbestanden | PI Expert Design Software Data | PI Expert Suite | Power Integrations |
| .UFD | Cellebrite | Gegevensbestanden | UFED Physical Analyzer Data | UFED Physical Analyzer | Cellebrite |
| .UFF | DataWave Technologies | Gegevensbestanden | DataWave SciWorks Physiological Data File | SciWorks | DataWave Technologies |
| .UFO | Ulead Systems | Gegevensbestanden | Ulead File Object | Ulead Photo Express | Ulead Systems |
| .UFS | iXsystems | Gegevensbestanden | FreeBSD Unix File System File | iXsystems PC-BSD | iXsystems |
| .UGA | Microsoft Corporation | Gegevensbestanden | Ulead GIF Animator File | GIF Animator | Microsoft Corporation |
| .UID | IBM | Gegevensbestanden | IBM VoiceType User Data File | IBM Voice Type | IBM |
| .UIL | IBM | Gegevensbestanden | AIX User Interface Language | IBM AIX | IBM |
| .UIN | Mail.Ru | Gegevensbestanden | ICQ 2001 Saved ICQ Contact Information | ICQ | Mail.Ru |
| .ULB | Micro Focus International plc | Gegevensbestanden | COBOL Unisys Library File | ACUCOBOL-GT | Micro Focus International plc |
| .ULF | MRF GeoSystems | Gegevensbestanden | Universal List File | MRF Mapping Tool Kit | MRF GeoSystems |
| .ULS | Microsoft Corporation | Gegevensbestanden | NetMeeting User Location Service File | Microsoft NetMeeting | Microsoft Corporation |
| .ULZ | Microsoft Corporation | Gegevensbestanden | Windows Updates Downloader Update List File | Windows Updates Downloader | Microsoft Corporation |
| .UM | Unknown Developer | Gegevensbestanden | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .UMC | Crestron Electronics | Gegevensbestanden | SIMPL Application | SIMPL Windows | Crestron Electronics |
| .UMF | GetMediaHolder.com | Gegevensbestanden | MediaHolder Universal Media Format File | MediaHolder | GetMediaHolder.com |
| .UMI | Creative Technology Ltd. | Gegevensbestanden | Creative PlayCenter CDDB Database File | Creative PlayCentre | Creative Technology Ltd. |
| .UND | DHI | Gegevensbestanden | MOUSE Urban Network Datafile | Unknown Software | Unknown Developer |
| .UNF | Open Source | Gegevensbestanden | UNIF ROM File | Nestopia | Open Source |
| .UNI | Open Source | Gegevensbestanden | MikMod UniMOD Module | CocoModX | Open Source |
| .UNIVERSAL3 | Unknown Developer | Gegevensbestanden | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .UNK | Shimadzu Scientific Instruments | Gegevensbestanden | UVProbe Data File | UVProbe | Shimadzu Scientific Instruments |
| .UNLOCK | Unknown Developer | Gegevensbestanden | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .UNQ | Swiftpage, Inc. | Gegevensbestanden | ACT! 3.0 Email Message Attachment | Act! | Swiftpage, Inc. |
| .UNT | Autodesk, Inc. | Gegevensbestanden | AutoCAD Unit Definition Data File | Autodesk AutoCAD | Autodesk, Inc. |
| .UNV | SAP | Gegevensbestanden | BusinessObjects Universe Data File | SAP BusinessObjects Business Intelligence | SAP |
| .UNX | Unknown Developer | Gegevensbestanden | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .UP | Apple | Gegevensbestanden | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .UPD | Various Developers | Gegevensbestanden | Program Update Information | Microsoft Notepad | Microsoft Corporation |
| .UPJ | Ulead Systems | Gegevensbestanden | Ulead Project File | Burn.Now | Corel |
| .UPL | StarMoney | Gegevensbestanden | StarMoney Agent Download File | StarMoney | StarMoney |
| .UPO | dBase, LLC | Gegevensbestanden | DBASE Compiled Update Data File | dBase | dBase, LLC |
| .UPR | Open Source | Gegevensbestanden | Fractal Parameters File | Apophysis | Open Source |
| .UPS | Open Source | Gegevensbestanden | UPS Patch File | MultiPatch patching tool | Open Source |
| .UPT | Open Source | Gegevensbestanden | Connectivity Memory Model Update Timing Input Data File | Neural Network Simulation Software | Open Source |
| .UPZ | StarMoney | Gegevensbestanden | StarMoney Agent Download File | StarMoney | StarMoney |
| .URD | Kinook Software, Inc. | Gegevensbestanden | Ultra Recall Database File | Ultra Recall | Kinook Software, Inc. |
| .URF | Matinsoft Inc. | Gegevensbestanden | AppFace Skin File | AppFace User Interface Development Kits | Matinsoft Inc. |
| .URI | Microsoft Corporation | Gegevensbestanden | List Of Uniform Resource Identifiers | Microsoft Notepad | Microsoft Corporation |
| .USA | Microsoft Corporation | Gegevensbestanden | American English Localization Language File | Microsoft Notepad | Microsoft Corporation |
| .USAGE | Unknown Developer | Gegevensbestanden | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .USE | Unknown Developer | Gegevensbestanden | Log File | Microsoft Notepad | Microsoft Corporation |
| .USL | Hewlett-Packard | Gegevensbestanden | LaserJet Landscape Font File | Classical Text Editor | Stefan Hagel |
| .USR | FileMaker Inc. | Gegevensbestanden | FileMaker Pro Database File | FileMaker Pro | FileMaker Inc. |
| .UST | UTAU Team | Gegevensbestanden | UTAU Sequence Text File | Vocal Synthesizer Tool UTAU | UTAU Team |
| .USTAR | Smith Micro Software | Gegevensbestanden | Uniform Standard Tape Archive Format File | Smith Micro StuffIt Deluxe | Smith Micro Software |
| .UTF | AOL | Gegevensbestanden | AOL Update Data | AOL | AOL |
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