| Extension | File Type | Files scanned | Contributor |
| U3D | Universal 3D | 3 | |
| U3P | U3 application Package | 5 | |
| UAE | UAE - WinUAE Configuration | 48 | Roberto 'rbnet' Bolli |
| UAP | WinCC User Archive export | 2 | |
| UBOX | Universe Sandbox simulation | 114 | Ryan Jones |
| UC2 | UltraCompressor 2 Archive | 13 | |
| UCCAPILOG | Microsoft UCC API Log | 4 | |
| UCE | UniCode Extensions | 16 | |
| UCF | Universal Communications Format | 4 | |
| UCI | Samsung YP-P2 theme | 16 | |
| UCS | Universal Classification Standard Database | 23 | |
| UCT | UC Browser Theme | 4 | |
| UDB | VBA32 Antivirus Signature | 7 | Scrapie |
| UDD | OllyDbg Module Info | 10 | |
| UDF | X-ray diffraction data file format | 18 | |
| UDS | NHTSA UDS-1992 crash test results | 375 | |
| UDS | Sierra Generations Family | 19 | |
| UDS | Unique Development Music | 5 | |
| UEF | Unified Emulator Format | 7 | |
| UEZ | Ulead COOL 3D (generic) | 9 | |
| UFA | UFA compressed archive | 10 | |
| UFI | UFOCaptureV2 Preset settings | 27 | |
| UFO | Ellisys Visual USB Data | 4 | |
| UFO | Ulead PhotoImpact Object(s) | 17 | |
| UHA | UHARC compressed archive | 3 | |
| UHS | Universal Hint System | 8 | |
| UI | Qt User Interface | 12 | |
| UIFILE | Windows Explorer UIFILE | 10 | |
| UIR | LabWindows User Interface Resource | 15 | |
| ULP | EAGLE script | 76 | Renan |
| UMD | UMD Photobook | 22 | |
| UMP | UModel Project | 47 | |
| UNI | UNIMod created by APlayer | 2 | |
| UNI | UNIMod module created by MikCvt | 2 | |
| UNR | Unreal Map | 31 | |
| UOF | Uniform Office Format | 5 | |
| UPC | Ultimate Paint Graphics Editor plugin/effect | 80 | |
| UPD | McAfee AV Pattern update | 21 | Chris Clemson |
| UPD | Ulead Private Data | 3 | |
| UPP | Unified Printer Parameter | 5 | |
| URF | AppFace skin | 31 | |
| URL | Windows URL shortcut | 126 | |
| USKN | KSDev ThemeEngine theme/skin | 8 | |
| USR | COREL Photo Paint User Defined Filter | 7 | |
| USR | EAGLE Parameter | 6 | |
| USR | USRobotics firmware | 6 | |
| USS | UAE saved state | 9 | |
| UTX | Unreal Texture | 13 | |
| UWF | UltraTracker Wave File audio | 20 | |
| UXDC | Office Data Retrieval Service Connection | 6 | |
| UZ1 | JB BAHN scenery | 6 | |
| UZ2 | JB BAHN scenery (Zoom2) | 8 | |
| UZ4 | JB BAHN scenery (Zoom4) | 7 | |
