| Extension | File Type | Files scanned | Contributor |
| U | KICK-Pascal Unit Interface | 13 | |
| U | Twist Update script | 13 | |
| U3D | Universal 3D | 3 | |
| U3P | U3 application Package | 5 | |
| U8 | Wii U8 archive | 2 | |
| UAE | UAE - WinUAE Configuration | 48 | Roberto 'rbnet' Bolli |
| UAEM | FS-UAE file metadata | 7 | |
| UAP | WinCC User Archive export | 2 | |
| UAR | UAR voicebank | 12 | |
| UASSET/U/UTX/UAX/UMX/UNR/UT3/UPK | UE3 Unreal Package (LE) | 214 | |
| UASSET/U/UTX/UAX/UMX/UNR/UT4/UPK | UE4 Unreal Package (LE) | 21 | |
| UBB/HNM | CRYO UBB video | 21 | |
| UBOX | Universe Sandbox simulation | 114 | Ryan Jones |
| UBZ | Open-Sankore document | 4 | |
| UC2 | UltraCompressor 2 Archive | 13 | |
| UCCAPILOG | Microsoft UCC API Log | 4 | |
| UCE | UniCode Extensions | 16 | |
| UCF | Universal Communications Format | 4 | |
| UCF | Xilinx User Constraints File | 10 | |
| UCI | Samsung YP-P2 theme | 16 | |
| UCLS | ObjectAid UML Explorer Class diagram | 15 | |
| UCM | Crazy Machines model | 150 | |
| UCS | Universal Classification Standard Database | 23 | |
| UCT | UC Browser Theme | 4 | |
| UDB | VBA32 Antivirus Signature | 7 | Scrapie |
| UDD | OllyDbg Module Info | 11 | |
| UDF | UYAP Document Format | 6 | |
| UDF | Universal Data Format | 6 | |
| UDF | Universal Data Format (lower case) | 11 | |
| UDF | X-ray diffraction data file format | 18 | |
| UDI | UDI Disk Image | 2 | |
| UDN | Alpha Four User Definition | 38 | |
| UDS | NHTSA UDS-1992 crash test results | 375 | |
| UDS | Sierra Generations Family | 19 | |
| UE-THEME | UltraEdit Theme | 19 | |
| UE2 | UltraCompressor 2 Encrypted archive | 5 | |
| UEB | UMT EFS Backup | 2 | |
| UEF | Unified Emulator Format | 9 | |
| UEW | UltraEdit Wordfile | 26 | |
| UEZ | Ulead COOL 3D (generic) | 9 | |
| UF2 | USB Flashing Format | 11 | |
| UFA | UFA compressed archive | 10 | |
| UFI | UFOCaptureV2 Preset settings | 27 | |
| UFL | Balko UFL game data archive | 2 | |
| UFM | Vermilion Desktop Composer music | 4 | |
| UFMF | Micro Fly Movie Format video | 2 | |
| UFNT | UltraDesign Font | 18 | |
| UFO | Ellisys Visual USB Data | 4 | |
| UFO | Ulead PhotoImpact Object(s) | 271 | |
| UFOZ | Unified Font Object (compressed) | 6 | |
| UFP | Ultimaker format package | 7 | |
| UFR | Ultra Fractal data and pixels | 6 | |
| UFX | Ultimate 3D Effect | 3 | |
| UGI/UGF | Universal Go Format | 14 | |
| UHA | UHARC compressed archive | 3 | |
| UHI | UHIC compressed bitmap (v1.0) | 2 | |
| UHI | UHIC compressed bitmap (v2.0) | 2 | |
| UHS | Universal Hint System (newer format) | 8 | |
| UHS | Universal Hint System (old 88a format) | 12 | |
| UHT | Dragon UnPACKer HTML Template | 3 | |
| UHU | UHO Software graphics | 2 | |
| UI | Qt User Interface | 21 | |
| UI | Sprint User Interface | 9 | |
| UIB | jetAudio theme | 5 | |
| UIFILE | Windows Explorer UIFILE | 10 | |
| UIR | LabWindows User Interface Resource | 15 | |
| UITXT | YSFlight localization info | 2 | |
| UJG | Lepton UJG bitmap | 2 | |
| ULP | EAGLE script | 76 | Renan |
| ULT | Ultra Tracker module | 8 | |
| ULX | Glulx Game | 6 | |
| ULZ | Ultra-fast LZ compressed data | 3 | |
| UMAP | Unreal Engine Map | 16 | |
| UMD | Canon printer User Mode backup | 2 | |
| UMD | UMD Photobook | 22 | |
| UMD/UAX | Unreal Engine 2.5 package | 6 | |
| UML | StarUML Model | 23 | |
| UML | StarUML Model (UTF-8) | 6 | |
| UMLCLASS_DIAGRAM | UML2Tools UML Class Diagram | 5 | |
| UMP | UModel Project | 47 | |
| UMV | UMV Video | 145 | |
| UMX | U-MYX song mix data | 29 | |
| UMX | Unreal Music | 28 | |
| UNF | Ulysses Native Format | 13 | |
| UNI | MikMod module | 9 | |
| UNI | UNIMod created by APlayer | 2 | |
| UNIF/UNF | Universal NES Image Format | 3 | |
| UNIT | High Speed Pascal compiled Unit | 36 | |
| UNITY3D | Unity Web Player scene | 10 | |
| UNITY3D | Unity Web Player scene (non compressed) | 7 | |
| UNITYPROJ | Unity 3D Project | 9 | |
| UNO | Useful Notes Note | 6 | |
| UNR | Unreal Map | 31 | |
| UOF | Uniform Office Format (generic) | 20 | |
| UOP | Uniform Office Format Presentation | 4 | |
| UOS | Uniform Office Format Spreadsheet | 7 | |
| UOT | Uniform Office Format Text document | 9 | |
| UP3 | UP Studio 3D model (CSM 3.0) | 17 | |
| UP3 | UP Studio 3D model (CSM 5.0) | 4 | |
| UPC | Ultimate Paint Graphics Editor plugin/effect | 80 | |
| UPD | Greaseweazle Update | 2 | |
| UPD | HxC Floppy Emulator firmware Update (Gotek) | 116 | |
| UPD | HxC Floppy Emulator firmware Update (generic) | 119 | |
| UPD | HxC Floppy Emulator firmware Update (slim) | 2 | |
| UPD | HxC Floppy Emulator firmware Update (std) | 2 | |
| UPD | McAfee AV Pattern update | 21 | Chris Clemson |
| UPD | Pioneer DJ CDJ-1000MK3 firmware | 4 | |
| UPD | Pioneer DJ CDJ-2000 firmware | 4 | |
| UPD | Pioneer DJ CDJ-900 firmware | 4 | |
| UPD | Pioneer DJ DDJ-1000 firmware | 3 | |
| UPD | Pioneer DJ DDJ-1000SRT firmware | 3 | |
| UPD | Pioneer DJ DDJ-800 firmware | 3 | |
| UPD | Pioneer DJ DDJ-ERGO firmware | 3 | |
| UPD | Pioneer DJ DDJ-FLX10 firmware | 3 | |
| UPD | Pioneer DJ DDJ-RB firmware | 3 | |
| UPD | Pioneer DJ DDJ-REV5 firmware | 3 | |
| UPD | Pioneer DJ DDJ-REV7 firmware | 3 | |
| UPD | Pioneer DJ DDJ-RR firmware | 3 | |
| UPD | Pioneer DJ DDJ-RZ firmware | 3 | |
| UPD | Pioneer DJ DDJ-RZX firmware | 3 | |
| UPD | Pioneer DJ DDJ-S1 firmware (USB) | 4 | |
| UPD | Pioneer DJ DDJ-S1 firmware (main) | 4 | |
| UPD | Pioneer DJ DDJ-SB firmware | 4 | |
| UPD | Pioneer DJ DDJ-SZ firmware | 4 | |
| UPD | Pioneer DJ DDJ-SZ2 firmware | 4 | |
| UPD | Pioneer DJ DJM-2000 firmware | 4 | |
| UPD | Pioneer DJ DJM-250MK2 firmware | 4 | |
| UPD | Pioneer DJ DJM-450 firmware | 3 | |
| UPD | Pioneer DJ DJM-750MK2 firmware | 3 | |
| UPD | Pioneer DJ DJM-850 firmware | 7 | |
| UPD | Pioneer DJ DJM-900NXS firmware | 4 | |
| UPD | Pioneer DJ DJM-S11 firmware | 4 | |
| UPD | Pioneer DJ DJM-S3 firmware | 3 | |
| UPD | Pioneer DJ DJM-S9 firmware | 3 | |
| UPD | Pioneer DJ DJM-T1 firmware | 3 | |
| UPD | Pioneer DJ TORAIZ AS-1 firmware | 3 | |
| UPD | Ulead Private Data | 3 | |
| UPD | dBASE 5.0 Update | 3 | |
| UPD | dBASE IV Update | 4 | |
| UPG | Pioneer DJ DVJ-X1 firmware | 3 | |
| UPLUGIN | Unreal Engine Plugin | 7 | |
| UPP | Unified Printer Parameter | 5 | |
| UPP | Unity Player Preferences | 9 | |
| UPROJECT | Unreal Engine Project | 10 | |
| UPS | VisualBoyAdvance UPS patch | 19 | |
| UPX | Ulead Photo Express image | 399 | |
| UQM | The Ur-Quan Masters game data archive | 4 | |
| URF | AppFace skin | 31 | |
| URF | Unirast bitmap | 2 | |
| URL | Windows URL shortcut | 126 | |
| USB | Ulysses Speaker Database | 15 | |
| USD | UML Sequence Diagram | 13 | |
| USDA/USD | Universal Scene Description (ASCII) | 654 | |
| USDC/USD | Universal Scene Description (binary) | 31 | |
| USDZ | Universal Scene Description Zipped AR format (USDA) | 4 | |
| USDZ | Universal Scene Description Zipped AR format (USDC) | 14 | |
| USDZ | Universal Scene Description Zipped AR format (generic) | 16 | |
| USEQ | USeq genome data | 7 | |
| USER | Visual Studio Project User Options | 8 | |
| USF | EVGA Precision X skin | 7 | |
| USF/USFLIB | USF Ultra64 Sound Format rip | 24 | |
| USG | Ultrabots/Xenobots Scenery | 17 | |
| USH | UCSHELL info | 5 | |
| USKN | KSDev ThemeEngine theme/skin | 9 | |
| USM | CRIWARE Sofdec2 Movie | 25 | Masamune3210 |
| USP | SIMCA Project | 4 | |
| USR | COREL Photo Paint User Defined Filter | 7 | |
| USR | EAGLE Parameter | 6 | |
| USR | USRobotics firmware | 6 | |
| USR | redshirt2 encrypted data | 5 | |
| USS | UAE Saved State | 15 | |
| UST | UTAU Sequence Text | 23 | |
| USTX | OpenUTAU Sequence Text | 22 | |
| USX | Unified Scripture Format XML | 93 | |
| USX | Unified Scripture Format XML (UTF8) | 136 | |
| UTI | SafeGuard PrivateCrypto Encrypted | 6 | |
| UTK | Maxis UTalk audio | 5 | |
| UTP | BinPatch patch | 2 | |
| UTX | Unreal Texture | 13 | |
| UU | UUencoded Base64 text | 8 | Joerg Jenderek |
| UUE/UU/XXE | UUencoded/XXencoded text | 495 | Joerg Jenderek |
| UV | Need for Speed 2 Video | 6 | |
| UV2 | uVision v2 Project | 26 | |
| UVN/UVD | Yamaha Tyros4 custom voice | 39 | |
| UVO | Sanyo Katana DLX call/voice memo | 6 | |
| UVOPT | uVision v4 Project Options | 31 | |
| UVOPTX | uVision v5 Project Options | 58 | |
| UVOX | Universal Voxel format | 16 | |
| UVPROJ | uVision v4 Project | 37 | |
| UVPROJX | uVision v5 Project | 54 | |
| UVW | UltraDesign drawing | 10 | |
| UWF | UltraTracker Wave File audio | 20 | |
| UWL/SAV/HWL | Corel Writing Tools User Word List | 27 | Joerg Jenderek |
| UXDC | Office Data Retrieval Service Connection | 6 | |
| UXF | UMLet diagram | 26 | |
| UZ1 | JB BAHN scenery | 6 | |
| UZ2 | JB BAHN scenery (Zoom2) | 8 | |
| UZ4 | JB BAHN scenery (Zoom4) | 7 | |
