| .u | Subsampled Raw YUV Bitmap Image |
| .u00 | The Price Is Right 2010 Game |
| .u01 | Barudan Embroidery |
| .u08 | UFile Data |
| .u10 | UFile 2010 Tax Return Format |
| .u11 | UFile 2011 Tax Return Format |
| .u12 | UFile 2012 Tax Return Format |
| .u255law | Exponential 8-bit Sound |
| .u2car | NFS:U2 Car |
| .u32 | Authorware Cover |
| .u3d | Universal 3D Format |
| .u3i | U3 Application Information Format |
| .u3p | U3 Platform Install Format |
| .u8 | Lemings 3D Audio Format |
| .u96 | EasyZip Temporary |
| .uae | WinUAE Configuration |
| .ual | Entrust Unattended Login Credentials |
| .uap | SIMATIC WinCC User Archive Export |
| .uaq | Microsoft Windows Action Queue |
| .uas | Ulead PhotoImpact Backup |
| .uav | OpenPilot UAV Settings |
| .uax | Unreal Audio Format |
| .ub | Unsigned Byte Audio Format |
| .ubifs | Linux UBIFS Image |
| .ubj | Universal Binary JSON Fiile |
| .ubm | IPhoto Mtwain Install |
| .ubox | Universe Sandbox Saved Data Format |
| .ubr | IIS WCAT Configuration |
| .uc | UnrealScript Class |
| .uc2 | UltraCompressor II Compressed Archive Format |
| .uccapilog | Microsoft Windows Live Messenger UCC API Log Format |
| .ucd | Prosa Diagram |
| .uce | Microsoft Windows XP System32 Format |
| .ucf | WebEx Universal Communications Format |
| .uci | UFS Explorer Backup Format |
| .ucl | Unreal Tournament Cache Format |
| .ucls | ObjectAid UML Explorer Class Diagram |
| .ucm | Urban Chaos Game Format |
| .ucp | SmartWare Communications Profile |
| .ucr | MicroSim PCBoard Unrouted Nets Report |
| .ucs | Homeworld Localized Unicode Strings |
| .ucsettings | Universal Converter Settings |
| .uct | UC Browser Theme Format |
| .ucv | Universal Converter Conversion |
| .ucx | Rational XDE |
| .ud | OmniPage User Dictionary |
| .uda | ZEMAX Model Data |
| .udb | Dynamics AX User Database Format |
| .udc | Adobe User Dictionary Format |
| .udcx | Microsoft Office Universal Data Connection Format |
| .udeb | Debian Package Format |
| .udf | Universal Disk Format |
| .udg | UDraw(Graph) |
| .udi | CorelDRAW Draw Script Textures Bitmap |
| .udid | Google Android Unique Device IDentifier |
| .udif | Apple Mac OS X Universal Disk Image Format |
| .udk | Unreal Map Format |
| .udl | Microsoft Data Link Format |
| .udm | MapView User Module |
| .udo | Moray UDO 3D |
| .udp | Corel VideoStudio Scene Information |
| .uds | FIMware Database Format |
| .udw | Raw Unsigned DoubleWord (32-bit) Data |
| .udx | PI Expert Design Software |
| .uea | Protector Suite QL Format |
| .ueed | Uconomix Encryption Engine Encrypted Directory |
| .uenc | UltraEdit Encrypted Text |
| .ues | Ulead Photo Explorer Slide Show |
| .uew | UltraEdit Wordfile |
| .uez | Ulead COOL 3D Graphics |
| .ufa | GIF Animator Animation Object Information |
| .ufc | SATURN Suite |
| .ufdr | UFED Physical Analyzer Report Package |
| .uff | DataWave SciWorks Physiological Data |
| .ufi | UFOCaptureV2 |
| .ufo | Ulead PhotoImpact Object Format |
| .ufp | Ulead Photo Project |
| .ufr | Ultra Fractal Format |
| .ufs | FreeBSD PC-BSD Unix File System Format |
| .ufs.uzip | FreeBSD Compressed Unix File System Format |
| .ufx | Ultimate 3D Game Engine Material |
| .ugd | Need For Speed Underground |
| .ugr | Apophysis Gradient |
| .uha | UHARC Compressed Archive |
| .uhh | Eurotherm Hydra History Encrypted |
| .uhs | Universal Hint System Binary |
| .uhtm | Unreal Tournament |
| .uhtml | UHTML Document Format |
| .ui | User Interface Format |
| .uibak | UltraISO Backup Disk Image |
| .uic | COCO Unit ICon |
| .uid | IBM Voice Type Users |
| .uid-zps | Zoner Photo Studio Picture Autobackup |
| .uif | MagicISO Universal Image Format |
| .uil | IBM AIX User Interface Language |
| .uin | ICQ 2001 Saved ICQ Contact Information |
| .uir | LabWindows User Interface Resource |
| .uis | WindowBlinds Personality Format |
| .uit | UIX XML Template |
| .uix | UiXML Page |
| .uiz | Spybot Search & Destroy Updater |
| .uk | English (British) Language Translation |
| .uka | Fifa World Cup Game Data |
| .uks | Microsoft Works |
| .ukx | Unreal Tournament Animation Format |
| .uky | UltraMixer Licence Key |
| .ulaw | Raw CCITT/ITU G.711 US Telephony Audio Format |
| .ulb | COBOL Unisys Library |
| .uld | Unified Luminaire Datafile |
| .ulf | Universal List Format |
| .ulp | EAGLE Script |
| .uls | NetMeeting User Location Service Format |
| .ult | Ultratracker Music Module (MOD) Format |
| .ulw | U-Law Audio Format |
| .ulys | Ulysses Project |
| .ulz | USBlyzer Data Capture Format |
| .umb | MemMaker Backup |
| .umd | UMD Mobile Ebook |
| .umf | MediaHolder Media Format |
| .uml | Visual UML Class Diagram & Model Format |
| .umlclass | Java UML Class Model Format |
| .umlx | Visual UML XML Model |
| .umm | Unified Molecular Modeling |
| .umod | Unreal Tournament MOD Installation Package Format |
| .ump | UModel Project |
| .umr | MyEclipse UML Model Repository |
| .ums | Rune 3D Mesh Format |
| .umx | Deus Ex Format |
| .una | SIMS Unauthorized Data |
| .unauth | SiteMinder Unauthorized Message Format |
| .uncompressed_size | Singularity Game |
| .unf | Btreive Unformatted |
| .uni | UniMOD/MikMOD Music Module Format |
| .unitxt | YS FLIGHT Language |
| .unity | Unity Scene Format |
| .unity3d | Unity Scene Web Player Format |
| .unitypackage | Unity Package Format |
| .unityproj | Unity 3D Project |
| .unit_variant | Total War Unit Variant Settings |
| .unix | Unix |
| .unk | UVProbe Format |
| .unknown | Unknown File Extension |
| .unl | Garmin MapSource Unlock Format |
| .unq | ACT! 3.0 Email Message Attachment |
| .unr | Unreal Map Format |
| .uns | Visual Navigation Suite Export Or Import Settings |
| .unt | AutoCAD Unit Definition Format |
| .unused | Photoline 4 Defaults |
| .unv | BusinessObjects Universe |
| .unx | Unix Text Format |
| .uof | Uniform Office Format |
| .uol | PhotoImpact Clipart Image |
| .uop | Unified Office Format Presentation Format |
| .uos | Unified Office Format Spreadsheet Format |
| .uot | Unified Office Format Text Format |
| .upc | Ultimate Paint Component |
| .upd | Update Format |
| .update | ISAPI Update Format |
| .updatr | IConomy Update Format |
| .updf | UltraPdf Publication |
| .upf | Nokia Phone Digital Photo Format |
| .upg | Firmware Upgrade Format |
| .upi | Ulead PhotoImpact Plugin |
| .upj | Ulead Project |
| .upk | Unreal 3 Game Data Package Format |
| .upk0 | Metro 2033 Game |
| .upl | StarMoney Agent Download |
| .upm | PenMail |
| .upo | DBASE Compiled Update Data |
| .upoi | IGO Points Of Interest Format |
| .upp | Ghostscript Uniprint Unified Printer Parameter |
| .upr | FileMaker User Spelling Dictionary |
| .ups | Microsoft Works |
| .upx | Ultimate Packer For EXecutable Format |
| .upz | StarMoney Agent Download |
| .uqm | The Ur-Quan Masters Archive Format |
| .urc | Unicenter Remote Control |
| .urd | Ultra Recall Database |
| .urd-journal | Ultra Recall Journal Update |
| .urf | AppFace Skin |
| .url | Internet Shortcut - URL - Uniform Resource Locator |
| .urls | GetRight Internet Shortcuts List |
| .url_ | Internet Shortcut - URL - Uniform Resource Locator |
| .urp | Universal Report |
| .urq | Review QuickChart Chart |
| .urr | Uniface Signatures Format |
| .urs | ControlDraw User Requirement Specifier |
| .urt | Utah Run-Length Encoded Bitmap Image Format |
| .us | English Language Translation |
| .us? | Microsoft Word Font |
| .usa | Microsoft Office Headers Format |
| .usb | SiSoftware Sandra |
| .usc | USC Module Definition |
| .usecasediagram | Microsoft Visual Studio Use Case Diagram |
| .useq | USeq Genome Data Format |
| .user | Microsoft Visual Studio Project User Options Format |
| .userprofile | Norton Antivirus User Profile Format |
| .usertile-ms | Microsoft Windows 8 Release Preview User Tile Format |
| .usf | Nintendo Ultra 64 Sound Format |
| .usflib | Nintendo 64 Song Library Format |
| .usg | PC-Doctor For Windows |
| .ush | Creston SIMPL Windows |
| .usi | UsiXML UI Description |
| .usl | ProWORX Nxt USL Loadable |
| .usm | USM Game Video Format |
| .usp | PageMaker Printer Font |
| .usr | FileMaker Pro Runtime Engine Database Application |
| .uss | WinUAE Saved State |
| .ust | WiFi SiStr Exported User Theme Format |
| .ustar | Uniform Standard Tape Archive Format |
| .usx | Unreal StaticMesh (hardware Brushes) Format |
| .ut2 | Unreal Tournament Map Format |
| .ut2mod | Unreal Tournament 2003 MOD Installation Package Format |
| .ut3 | Unreal Tournament 3 Map Format |
| .ut4mod | Unreal Tournament 2004 MOD Installation Package Format |
| .ut8 | Ghost Master Language Format |
| .utb | µTBackup Backup |
| .utc | BioWare Aurora Game Engine Creature Format |
| .utd | BioWare Aurora Game Engine Door |
| .ute | BioWare Aurora Game Engine Encounter |
| .utf8 | Unicode UTF-8 Encoded Text |
| .uti | Spybot - Search & Destroy Includes |
| .utl | QuickBooks Data |
| .utm | BioWare Aurora Game Engine Store |
| .utp | BioWare Aurora Game Engine Placeable Object |
| .utpart | µTorrent Unfinished Download |
| .uts | Spybot - Search & Destroy Include |
| .utt | BioWare Aurora Engine Trigger |
| .utw | Unity Whirld Format |
| .utx | Unreal Texture Format |
| .utxt | Unicode Text Format |
| .utz | UIQ Theme Package |
| .uu | Uuencoded (compressed) File Archive (ascii) |
| .uud | UUDecoded Format |
| .uue | UUEncoded Format |
| .uus | Ultra Utility Skin |
| .uv2 | µVision Project |
| .uvcab | UVScreenCamera Films |
| .uvf | Uncertain Value Format |
| .uvnts | UVScreenCamera Notes |
| .uvproj | µVision4 Project |
| .uvr | UVR Mesh |
| .uvs | UVScreenCamera Segments |
| .uvseg | UVScreenCamera Segments |
| .uvw | 3D Studio MAX UVW Coordinates |
| .uvx | Unreal Tournament Saved Game |
| .uw | Unsigned Word (16-bit) Music Format |
| .uwf | UltraTracker WaveSample Music Format |
| .uwl | Corel WordPerfect User Word List Format |
| .uxdc | Microsoft Excel Data Retrieval Service Connection |
| .uxf | UMLet UML EXchange Format |
| .uxx | Unreal Tournament 2003 Cache Format |
| .uxy | Barudan Embroidery Outline |
| .uyvy | YUV 16-bit Image Format |
| .uz2 | Unreal Tournament Compressed |
| .uze | UltimateZip Compressed Archive |
| .uzip | FreeBSD Compressed Disk Image Format |
| .uzy | Backdoor.Sokacaps |
File extensions are not normally displayed to users. To change this, go to Control Panel, select Appearance and Personalization and Folder Options. Next, select the View and find the option "Hide extensions for known file types". The option should be deselected (cleared) and confirmed with OK.
