| File extension | Developer/Producer | File description |
|---|---|---|
| U file extension | N/A | Unreal Tournament System Format |
| U00 file extension | Ludia Inc. | The Price Is Right 2010 Game |
| U01 file extension | Barudan | Barudan Embroidery |
| U08 file extension | Dr Tax Software | UFile Data |
| U10 file extension | Dr Tax Software | UFile 2010 Tax Return Format |
| U11 file extension | Dr Tax Software | UFile 2011 Tax Return Format |
| U12 file extension | Dr Tax Software | UFile 2012 Tax Return Format |
| U255LAW file extension | N/A | Exponential 8-bit Sound |
| U2CAR file extension | N/A | NFS:U2 Car |
| U32 file extension | Adobe Systems Incorporated | Authorware Cover |
| U3D file extension | 3D Industry Forum | Universal 3D Format |
| U3I file extension | N/A | U3 Application Information Format |
| U3P file extension | N/A | U3 Platform Install Format |
| U8 file extension | Clockwork Games | Lemings 3D Audio Format |
| U96 file extension | ConeXware, Inc. | EasyZip Temporary |
| UAE file extension | N/A | WinUAE Configuration |
| UAL file extension | Entrust | Entrust Unattended Login Credentials |
| UAP file extension | Siemens AG | SIMATIC WinCC User Archive Export |
| UAQ file extension | Microsoft Corporation | Microsoft Windows Action Queue |
| UAS file extension | Ulead Systems | Ulead PhotoImpact Backup |
| UAV file extension | OpenPilot Foundation | OpenPilot UAV Settings |
| UAX file extension | N/A | Unreal Audio Format |
| UB file extension | N/A | Unsigned Byte Audio Format |
| UBIFS file extension | N/A | Linux UBIFS Image |
| UBJ file extension | The Buzz Media | Universal Binary JSON Fiile |
| UBM file extension | N/A | IPhoto Mtwain Install |
| UBOX file extension | Universe Sandbox | Universe Sandbox Saved Data Format |
| UBR file extension | Microsoft Corporation | IIS WCAT Configuration |
| UC file extension | N/A | UnrealScript Class |
| UC2 file extension | N/A | UltraCompressor II Compressed Archive Format |
| UCCAPILOG file extension | Microsoft | Microsoft Windows Live Messenger UCC API Log Format |
| UCD file extension | Insoft Oy. | Prosa Diagram |
| UCE file extension | Microsoft | Microsoft Windows XP System32 Format |
| UCF file extension | Cisco Systems | WebEx Universal Communications Format |
| UCI file extension | SysDevSoftware | UFS Explorer Backup Format |
| UCL file extension | Tripwire Interactive | Killing Floor System Format |
| UCLS file extension | ObjectAid LLC | ObjectAid UML Explorer Class Diagram |
| UCM file extension | Viva Media | Urban Chaos Game Format |
| UCP file extension | SmartWare Corporation | SmartWare Communications Profile |
| UCR file extension | Cadence Design Systems, Inc. | MicroSim PCBoard Unrouted Nets Report |
| UCS file extension | Relic Entertainment Inc. | Homeworld Localized Unicode Strings |
| UCSETTINGS file extension | Anthony R. Daly | Universal Converter Settings |
| UCT file extension | UC Mobile | UC Browser Theme Format |
| UCV file extension | Anthony R. Daly | Universal Converter Conversion |
| UCX file extension | IBM | Rational XDE |
| UD file extension | Nuance Communications | OmniPage User Dictionary |
| UDA file extension | ZEMAX Development Corporation | ZEMAX Model Data |
| UDB file extension | Microsoft | Dynamics AX User Database Format |
| UDC file extension | Adobe Systems | Adobe User Dictionary Format |
| UDCX file extension | Microsoft | Microsoft Office Universal Data Connection Format |
| UDEB file extension | The Debian Project | Debian Package Format |
| UDF file extension | Optical Storage Technology Association | Universal Disk Format |
| UDG file extension | Universität Bremen | UDraw(Graph) |
| UDI file extension | Corel Corporation | CorelDRAW Draw Script Textures Bitmap |
| UDID file extension | Google Inc. | Google Android Unique Device IDentifier |
| UDIF file extension | Apple, Inc. | Apple Mac OS X Universal Disk Image Format |
| UDK file extension | Epic Games | Unreal Map Format |
| UDL file extension | Microsoft | Microsoft Data Link Format |
| UDM file extension | MotCreations.com | MapView User Module |
| UDO file extension | N/A | Moray UDO 3D |
| UDP file extension | Corel Corporation | Corel VideoStudio Scene Information |
| UDS file extension | HSRA | Uniform Data System Format |
| UDW file extension | N/A | Raw Unsigned DoubleWord (32-bit) Data |
| UDX file extension | Power Integrations | PI Expert Design Software |
| UEA file extension | AuthenTec | Protector Suite QL Format |
| UEED file extension | Uconomix Technologies | Uconomix Encryption Engine Encrypted Directory |
| UENC file extension | IDM Computer Solutions, Inc. | UltraEdit Encrypted Text |
| UES file extension | Corel Corporation | Ulead Photo Explorer Slide Show |
| UEW file extension | IDM Computer Solutions, Inc. | UltraEdit Wordfile |
| UEZ file extension | Ulead Systems | Ulead COOL 3D Graphics |
| UFA file extension | N/A | GIF Animator Animation Object Information |
| UFC file extension | Atkins-ITS Transport Software | SATURN Suite |
| UFDR file extension | Cellebrite Mobile Synchronization LTD. | UFED Physical Analyzer Report Package |
| UFF file extension | DataWave Technologies | DataWave SciWorks Physiological Data |
| UFI file extension | SonotaCo | UFOCaptureV2 |
| UFO file extension | Ulead | Ulead PhotoImpact Object Format |
| UFP file extension | Corel Corporation | Ulead Photo Project |
| UFR file extension | Frederik Slijkerman | Ultra Fractal Format |
| UFS file extension | The FreeBSD Project | FreeBSD PC-BSD Unix File System Format |
| UFSUZIP file extension | The FreeBSD Project | FreeBSD Compressed Unix File System Format |
| UFX file extension | Ultimate 3D | Ultimate 3D Game Engine Material |
| UGD file extension | Electronic Arts, Inc. | Need For Speed Underground |
| UGR file extension | Mark Townsend | Apophysis Gradient |
| UHA file extension | UHarc | UHARC Compressed Archive |
| UHH file extension | Eurotherm UK | Eurotherm Hydra History Encrypted |
| UHS file extension | Jason Strautman | Universal Hint System Binary |
| UHTM file extension | Epic Games, Inc. | Unreal Tournament |
| UHTML file extension | UC Mobile | UC Browser Saved Webpage |
| UI file extension | Nokia | User Interface Format |
| UIBAK file extension | EZB Systems | UltraISO Backup Disk Image |
| UIC file extension | N/A | COCO Unit ICon |
| UID file extension | IBM | IBM Voice Type Users |
| UID-ZPS file extension | ZONER, Inc. | Zoner Photo Studio Picture Autobackup |
| UIF file extension | MagicISO | MagicISO Universal Image Format |
| UIL file extension | IBM | IBM AIX User Interface Language |
| UIN file extension | ICQ LLC | ICQ 2001 Saved ICQ Contact Information |
| UIR file extension | National Instruments Corporation | LabWindows User Interface Resource |
| UIS file extension | Stardock | WindowBlinds Personality Format |
| UIT file extension | N/A | UIX XML Template |
| UIX file extension | Oracle Corporation | UiXML Page |
| UIZ file extension | Safer-Networking Ltd. | Spybot Search & Destroy Updater |
| UK file extension | N/A | English (British) Language Translation |
| UKA file extension | Electronic Arts, Inc. | Fifa World Cup Game Data |
| UKS file extension | Microsoft Corporation | Microsoft Works |
| UKX file extension | Epic Games | Unreal Tournament Animation Format |
| UKY file extension | UltraMixer Digital Audio Solutions | UltraMixer Licence Key |
| ULAW file extension | N/A | Raw CCITT/ITU G.711 US Telephony Audio Format |
| ULB file extension | N/A | COBOL Unisys Library |
| ULD file extension | DIAL GMBH | Unified Luminaire Datafile |
| ULF file extension | N/A | Universal List Format |
| ULP file extension | CadSoft Computer, Inc. | EAGLE Script |
| ULS file extension | Microsoft | NetMeeting User Location Service Format |
| ULT file extension | N/A | Ultratracker Music Module (MOD) Format |
| ULW file extension | N/A | U-Law Audio Format |
| ULYS file extension | The Soulmen | Ulysses Project |
| ULZ file extension | USBlyzer | USBlyzer Data Capture Format |
| UMB file extension | Microsoft Corporation | MemMaker Backup |
| UMD file extension | jumd team | UMD Mobile Ebook |
| UMF file extension | GetMediaHolder.com | MediaHolder Media Format |
| UML file extension | N/A | Visual UML Class Diagram & Model Format |
| UMLCLASS file extension | N/A | Java UML Class Model Format |
| UMLX file extension | Visual Object Modelers, Inc. | Visual UML XML Model |
| UMM file extension | N/A | Unified Molecular Modeling |
| UMOD file extension | Epic Games | Unreal Tournament MOD Installation Package Format |
| UMP file extension | Altova | UModel Project |
| UMR file extension | Genuitec, LLC | MyEclipse UML Model Repository |
| UMS file extension | Human Head Studios | Rune 3D Mesh Format |
| UMX file extension | Epic Games | Unreal Tournament Music Package |
| UNA file extension | Capita Business Services Ltd. | SIMS Unauthorized Data |
| UNAUTH file extension | Netegrity | SiteMinder Unauthorized Message Format |
| UNCOMPRESSED_SIZE file extension | Raven Software | Singularity Game |
| UNF file extension | Novell, Inc. | Btreive Unformatted |
| UNI file extension | N/A | UniMOD/MikMOD Music Module Format |
| UNITXT file extension | N/A | YS FLIGHT Language |
| UNITY file extension | Unity Technologies | Unity Scene Format |
| UNITY3D file extension | Unity Technologies | Unity Scene Web Player Format |
| UNITYPACKAGE file extension | Unity Technologies | Unity Package Format |
| UNITYPROJ file extension | Unity Technologies | Unity 3D Project |
| UNIT_VARIANT file extension | The Creative Assembly | Total War Unit Variant Settings |
| UNIX file extension | The Open Group | Unix |
| UNK file extension | N/A | Unknown Format |
| UNKNOWN file extension | N/A | Unknown File Extension |
| UNL file extension | Garmin | Garmin MapSource Unlock Format |
| UNQ file extension | The Sage Group plc | ACT! 3.0 Email Message Attachment |
| UNR file extension | N/A | Unreal Map Format |
| UNS file extension | Nobeltec, Inc. | Visual Navigation Suite Export Or Import Settings |
| UNT file extension | Autodesk | AutoCAD Unit Definition Format |
| UNUSED file extension | Impressum | Photoline 4 Defaults |
| UNV file extension | SAP AG | BusinessObjects Universe |
| UNX file extension | N/A | Unix Text Format |
| UOF file extension | N/A | Uniform Office Format |
| UOL file extension | Ulead Systems | PhotoImpact Clipart Image |
| UOP file extension | Electronic Arts | Ultima Online Patch Format |
| UOS file extension | N/A | Unified Office Format Spreadsheet Format |
| UOT file extension | N/A | Unified Office Format Text Format |
| UPC file extension | J-T-L Development Inc. | Ultimate Paint Component |
| UPDATE file extension | EggCentric | ISAPI Update Format |
| UPDATR file extension | Nijikokun | IConomy Update Format |
| UPDF file extension | Software995 | UltraPdf Publication |
| UPF file extension | Bentley Systems | MicroStation User Preference Format |
| UPG file extension | N/A | Firmware Upgrade Format |
| UPI file extension | N/A | Ulead PhotoImpact Plugin |
| UPJ file extension | N/A | Ulead Project |
| UPK file extension | Epic Games | Unreal 3 Game Data Package Format |
| UPK0 file extension | 4A Games | Metro 2033 Game |
| UPL file extension | Star Finanz GmbH | StarMoney Agent Download |
| UPM file extension | UC-Logic Technology Corp. | PenMail |
| UPO file extension | dataBased Intelligence, Inc. | DBASE Compiled Update Data |
| UPOI file extension | Nav N Go | IGO Points Of Interest Format |
| UPP file extension | Artifex Software | Ghostscript Uniprint Unified Printer Parameter |
| UPR file extension | FileMaker | FileMaker User Spelling Dictionary |
| UPS file extension | Microsoft Corporation | Microsoft Works |
| UPX file extension | Markus Oberhumer | Ultimate Packer For EXecutable Format |
| UPZ file extension | Star Finanz GmbH | StarMoney Agent Download |
| UQM file extension | The Ur-Quan Masters | The Ur-Quan Masters Archive Format |
| URC file extension | CA Technologies | Unicenter Remote Control |
| URD file extension | Kinook Software, Inc. | Ultra Recall Database |
| URD-JOURNAL file extension | Kinook Software, Inc. | Ultra Recall Journal Update |
| URF file extension | Matinsoft Inc. | AppFace Skin |
| URL file extension | N/A | Internet Shortcut - URL - Uniform Resource Locator |
| URLS file extension | Headlight Software, Inc. | GetRight Internet Shortcuts List |
| URL_ file extension | N/A | Internet Shortcut - URL - Uniform Resource Locator |
| URP file extension | Universal Software C. T. | Universal Report |
| URQ file extension | Eurotherm UK | Review QuickChart Chart |
| URR file extension | Compuware | Uniface Signatures Format |
| URS file extension | ControlDraw Ltd. | ControlDraw User Requirement Specifier |
| URT file extension | N/A | Utah Run-Length Encoded Bitmap Image Format |
| US file extension | N/A | English Language Translation |
| US? file extension | Microsoft Corporation | Microsoft Word Font |
| USA file extension | Epic Games | Unreal Saved Game Format |
| USB file extension | SiSoftware | SiSoftware Sandra |
| USC file extension | Analogue Process Control Services | USC Module Definition |
| USECASEDIAGRAM file extension | Microsoft Corporation | Microsoft Visual Studio Use Case Diagram |
| USEQ file extension | Huntsman Cancer Institute | USeq Genome Data Format |
| USER file extension | Microsoft | Microsoft Visual Studio Project User Options Format |
| USERPROFILE file extension | Symantec | Norton Antivirus User Profile Format |
| USERTILE-MS file extension | Microsoft | Microsoft Windows 8 Release Preview User Tile Format |
| USF file extension | N/A | Nintendo Ultra 64 Sound Format |
| USFLIB file extension | N/A | Nintendo 64 Song Library Format |
| USG file extension | PC-Doctor, Inc. | PC-Doctor For Windows |
| USH file extension | Creston Electronics, Inc. | Creston SIMPL Windows |
| USI file extension | N/A | UsiXML UI Description |
| USL file extension | Schneider Electric S.A. | ProWORX Nxt USL Loadable |
| USM file extension | N/A | USM Game Video Format |
| USP file extension | Adobe Systems Incorporated | PageMaker Printer Font |
| USR file extension | FileMaker | FileMaker Pro Runtime Engine Database Application |
| USS file extension | N/A | WinUAE Saved State |
| UST file extension | UTAU | UTAU Sequence Format |
| USTAR file extension | N/A | Uniform Standard Tape Archive Format |
| USX file extension | N/A | Unreal StaticMesh (hardware Brushes) Format |
| UT2 file extension | Epic Games | Unreal Tournament Map Format |
| UT2MOD file extension | Epic Games | Unreal Tournament 2003 MOD Installation Package Format |
| UT3 file extension | Epic Games | Unreal Tournament 3 Map Format |
| UT4MOD file extension | Epic Games | Unreal Tournament 2004 MOD Installation Package Format |
| UT8 file extension | Sick Puppies | Ghost Master Language Format |
| UTB file extension | N/A | µTBackup Backup |
| UTC file extension | BioWare | BioWare Aurora Game Engine Creature Format |
| UTD file extension | BioWare | BioWare Aurora Game Engine Door |
| UTE file extension | BioWare | BioWare Aurora Game Engine Encounter |
| UTF8 file extension | N/A | Unicode UTF-8 Encoded Text |
| UTI file extension | Safer-Networking Ltd. | Spybot - Search & Destroy Includes |
| UTL file extension | Intuit Inc. | QuickBooks Data |
| UTM file extension | BioWare | BioWare Aurora Game Engine Store |
| UTP file extension | BioWare | BioWare Aurora Game Engine Placeable Object |
| UTPART file extension | BitTorrent, Inc. | µTorrent Unfinished Download |
| UTS file extension | Safer-Networking Ltd. | Spybot - Search & Destroy Include |
| UTT file extension | BioWare | BioWare Aurora Engine Trigger |
| UTW file extension | Aubrey Falconer | Unity Whirld Format |
| UTX file extension | N/A | Unreal Texture Format |
| UTZ file extension | UIQ Technology | UIQ Theme Package |
| UU file extension | N/A | Uuencoded (compressed) File Archive (ascii) |
| UUD file extension | N/A | UUDecoded Format |
| UUE file extension | N/A | UUEncoded Format |
| UUS file extension | BobaFett | Ultra Utility Skin |
| UV2 file extension | ARM Holdings plc | µVision Project |
| UVCAB file extension | UVsoftium | UVScreenCamera Films |
| UVF file extension | N/A | Uncertain Value Format |
| UVNTS file extension | UVsoftium | UVScreenCamera Notes |
| UVPROJ file extension | ARM Holdings plc | µVision4 Project |
| UVR file extension | N/A | UVR Mesh |
| UVS file extension | UVsoftium | UVScreenCamera Segments |
| UVSEG file extension | N/A | UVScreenCamera Segments |
| UVW file extension | Autodesk, Inc. | 3D Studio MAX UVW Coordinates |
| UVX file extension | Epic Games | Unreal Tournament Saved Game |
| UW file extension | N/A | Unsigned Word (16-bit) Music Format |
| UWF file extension | N/A | UltraTracker WaveSample Music Format |
| UWL file extension | Corel | Corel WordPerfect User Word List Format |
| UXDC file extension | Microsoft Corporation | Microsoft Excel Data Retrieval Service Connection |
| UXF file extension | UMLet Team | UMLet UML EXchange Format |
| UXX file extension | N/A | Unreal Tournament 2003 Cache Format |
| UXY file extension | Barudan | Barudan Embroidery Outline |
| UYVY file extension | N/A | YUV 16-bit Image Format |
| UZ2 file extension | Epic Games, Inc. | Unreal Tournament Compressed |
| UZE file extension | SWE von Schleusen | UltimateZip Compressed Archive |
| UZIP file extension | Maxim Sobolev | FreeBSD Compressed Disk Image Format |
| UZY file extension | N/A | Backdoor.Sokacaps |
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