| .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 |
