| ucs | F5 User Configuration Set |
| uct | UC Browser Theme |
| ud | OmniPage User Dictionary |
| uda | Zemax User-defined Aperture File |
| udb | Microsoft Dynamics AX User Database |
| udc | Adobe InDesign User Dictionary |
| udf | Windows NT Uniqueness Database File |
| udi | PANalytical Peak Data |
| udid | Android Unique Device Identifier Data |
| udl | MDAC Universal Data Link |
| udp | Corel VideoStudio Scene Information |
| uds | Generations Family Tree Document |
| udt | wyUpdate Update Details File |
| udt | Visilogic Utilities Data Table |
| udx | Philips Xenium Mobile Contacts |
| udz | UDA ConstructionSuite Data |
| uef | Unified Emulator Format |
| uel | Tekla Structures Custom Components Data |
| ues | Ulead Photo Explorer SlideShow Data |
| uew | UltraEdit Word File |
| ufc | SATURN Suite Data |
| ufd | UFED Physical Analyzer Data |
| ufdr | UFED Physical Analyzer Report Archive |
| ufdx | UFED Physical Analyzer Extraction Metadata |
| uff | BrainVoyager QX User Defined File Format |
| ufi | User Flash Image |
| ufl | Verity Server Custom Fields File |
| ufo | SNES ROM Image UFO |
| ufo | PhotoImpact Object |
| uft | Torque Font Cache |
| ufx | Ultimate 3D Effects Data |
| ugf | Go Game Data |
| ugi | Go Game Data |
| ugp | Aroma Unit Genome Position Data |
| ugr | Apophysis Gradient File |