| uht | Amulet uHTML GUI Data |
| ui | QT User Interface File |
| uic | COCO Unit ICon File |
| uic | Aveva CAF User Interface Customization File |
| uih | Geoworks UI Compiler Espire Header |
| uir | NI LabWindows User Interface Resource File |
| uit | UIX XML Page Template |
| uix | UIX XML Page |
| ulb | COBOL Unisys Library File |
| ulc | RevaSoft ULC Application |
| ulp | EAGLE User Language Program |
| umc | Crestron SIMPL User Module |
| uml | Unified Modeling Language File |
| uml | StarUML v1 Model Data |
| umlf | StarUML UML Fragment Data |
| upkg | Unreal Enigine Package Flags |
| upo | DBase Compiled Update Data |
| ur | Ur Source Code |
| urf | ProWorx NXT User Configurable Register Editor Data |
| urp | Universal Report Document |
| urr | Uniface Runtime Repository |
| urs | Ur Signature |
| ush | Crestron SIMPL CNX Compile |
| usi | UsiXML UI Description Data |
| uskn | KSDev ThemeEngine Theme |
| usl | ProWorx NXT USL Loadable |
| uwc | Laird smartBASIC Compiled Bytecode |
| uxo | RapidPLUS XML User Defined Object |
| v | Coq Source Code |
| v | Verilog HDL Source |
| v2z | TI Voyage 200 Assembly Program |
| v3s | Visual3D Pipeline |
| v4p | VVVV Patchlet |
| vala | Vala Source Code File |
| vbg | Visual Basic Project Group |