U
| Ext. | Description | Used by |
|---|---|---|
| U3D | Model file format | Ultimate 3D |
| U3D | Universal 3D | Adobe Acrobat |
| U8 | Sample | |
| UB | Raw Unsigned Byte (8bit) data | AWAVE |
| UC2 | Archive | UC II |
| UCN | New archive | UC II |
| UCS | Unknownsoft Service Pack (mainly for Shadow OS 2009) | Unknownsoft Computer Systems |
| UDF | Filter | Aldus Photostyler |
| UDO | Data | Moray User Defined Object |
| UDIWWW | bookmark data | UdiWWW |
| UDW | Raw Unsigned DWord data | AWAVE |
| UE2 | Encrypted archive | UC II |
| UFI | data | UFO CaptureV2 settings - |
| UFO | graphic file | Ulead Photo Impact |
| UHA | UHarc archive | |
| UHS | Universal Hint System (binary file) | |
| UI | Espire source code file | Geoworks UI Compiler Geos |
| UI | Qt Designer's UI File | Trolltech Qt Designer |
| UI | User interface | Sprint |
| UIF | Long prompts for windows | WordPerfect for Win |
| UIH | Espire header file | Geoworks UI Compiler Geos |
| UL | ULAW audio file | |
| ULD | Information about uploaded files | Procomm Plus - Datastorm Technologies, Inc. |
| ULF | data | MRF Mapping Tool Kit Universal List Format |
| ULT | Music | UltraTracker |
| UMB | Backup file | MemMaker |
| UNF | Universal NES Image Interchange File Format | |
| UNI | Datafile | Forecast Pro |
| UNI | Music format | UNIMOD - MIKMOD |
| UNIF | Universal NES Image Interchange File Format | |
| UNV | Text file containing finite elements nodal coordinates and more See notes | Originally used by SDRC for its I-deas software; a lot of simulation software use it today |
| UNX | Text file containing UNIX specific info | |
| UOML | UniqueObject Markup Language (UOML) is a XML-based markup language | |
| UOP | Uniform Office Format presentation | |
| UOS | Uniform Office Format spreadsheet | |
| UOT | Uniform Office Format text | |
| UPD | Update data | dBASE |
| UPO | Compiled update data | dBASE |
| URL | Internet Shortcut (URL) | Windows Explorer |
| URT | Utah Raster Toolkit | |
| USE | Log file | |
| USP | Printer font with USASCII extended character set | Adobe PageMaker |
| USR | User database file | Procomm Plus - Turbo C++ tour |
| UST | UTAU vocal track | UTAU |
| UT! | datafile | uTorrent |
| UTZ | Mobile Phone Theme | Sony Ericsson |
| UU | Encoded file (ASCII) | UUENCODE/UUDECODE - Uuencoding |
| UUE | Encoded file (ASCII) | UUENCODE/UUDECODE - Uuencoding |
| UW | Raw Unsigned Word 16bit data | AWAVE |
| UWF | UltraTracker WaveSample | UltraTracker |
Read more about this topic: List Of File Formats (alphabetical)