| u11 | UFile 2011 Tax Return Data |
| u12 | UFile 2012 Tax Return Data |
| u2n | Unicode to Native Mapping Data |
| u32 | Adobe Authorware Cover File |
| ual | Entrust Auto-enrollment Server Unattended Login Credentials |
| uamodel | OPC-UA Model Data |
| uap | SIMATIC WinCC User Archive Export |
| ubj | Universal Binary JSON Data |
| ubjson | Universal Binary JSON Data |
| ubox | Universe Sandbox Simulation Data |
| ubs | Microsoft BugsLayer Data |
| ubu | UEFI BIOS Updater Data |
| ucc | National UCC Fininacing Statement Data |
| uccapilog | Windows Live Messenger UCC API Log |
| ucd | AVS Unstructured Cell Data |
| uce | Microsoft Windows Unicode Extensions Data |
| ucf | Xilinx User Constraints File |
| uci | UFS Explorer Compressed Image |
| ucn | Unisys U-ACT Configurator Data |
| ucr | MicroSim PCBoard Unrouted Nets Report Data |
| 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 |
| uddf | Universal Dive Data Format Data |
| udeb | Debian Package |
| udf | Windows NT Uniqueness Database File |
| udi | PANalytical Peak Data |
