| U | Unreal Tournament Class Package | Game Files | 6 | 3 |
| U2 | Telelogic Tau UML Model File | Developer Files | 1 | 1 |
| U2C | Uncommon File Extension | Uncommon Files | 1 | 1 |
| U2E | Unreal Game Translation |
Uncommon Files
| 1 | 1 |
| U2P | Telelogic Tau UML Program Text File |
Developer Files
| 1 | 1 |
| U3D | Universal 3D File | 3D Image Files | 6 | 1 |
| U3I | U3 Application Information File | Text Files | 1 | 1 |
| U3P | U3 Platform Install Data File | Executable Files | 2 | 1 |
| U4A | UnInstaller Archive File |
Executable Files
| 1 | 1 |
| U8 | Mario Kart Wii Archive File |
Game Files
| 3 | 3 |
| U96 | EasyZip Temporary File | Temporary Files | 1 | 1 |
| U98 | USA 98 Streets & Destinations Tour Data File | Data Files | 1 | 1 |
| UAD | Uncommon File Extension |
Uncommon Files
| 1 | 1 |
| UAE | WinUAE Configuration Data |
Developer Files
| 1 | 1 |
| UAP | SIMATIC WinCC User Archive Export | Compressed Files | 2 | 2 |
| UAS | Ulead PhotoImpact Backup File | Backup Files | 1 | 1 |
| UAX | Unreal Audio Package | Audio Files | 4 | 1 |
| UB | Unsigned Byte Audio File |
Audio Files
| 4 | 1 |
| UB1 | X-Driven DRS-1100 Recorded Video | Video Files | 1 | 1 |
| UBB | BEA eLink Adapter Configuration Data | Settings Files | 1 | 1 |
| UBF | MXT3010 Universal Boot Format |
Data Files
| 1 | 1 |
| UBI | Ubisoft Logo Animation |
Game Files
| 1 | 1 |
| UBIFS | Linux UBIFS Image | Disk Image Files | 1 | 1 |
| UBJ | Universal Binary JSON File |
Data Files
| 4 | 1 |
| UBK | National Construction Estimator 32 User Costbook File |
Data Files
| 2 | 2 |
| UBL | DM35x User Boot Loader Data | System Files | 1 | 1 |
| UBM | IPhoto Mtwain Install File |
Uncommon Files
| 1 | 1 |
| UBOX | Universe Sandbox Saved Data |
Data Files
| 1 | 1 |
| UBS | Tracesrv Bugslayer Utility File |
Uncommon Files
| 1 | 1 |
| UBX | U-center UBX GPS Log Data | GIS Files | 1 | 1 |
| UBZ | Open-Sankore Document |
Data Files
| 1 | 1 |
| UC | UnrealScript Class |
Game Files
| 1 | 1 |
| UC2 | UltraCompressor 2 Archive |
Compressed Files
| 1 | 1 |
| UCA | Unicode Collation Algorithm Data File |
Uncommon Files
| 1 | 1 |
| UCC | Uniform Commercial Code Form File |
Data Files
| 1 | 1 |
| UCCAPILOG | Microsoft UCC API Log File |
Data Files
| 1 | 1 |
| UCD | Prosa Diagram | Graphic Files | 1 | 1 |
| UCE | Windows UCE System File |
System Files
| 1 | 1 |
| UCF | WebEx Media File | Web Files | 3 | 3 |
| UCG | CG Infinity Graphic |
Graphic Files
| 1 | 1 |
| UCI | UFS Explorer Backup File |
Backup Files
| 2 | 2 |
| UCL | Killing Floor System File |
Game Files
| 1 | 1 |
| UCLS | ObjectAid UML Explorer Class Diagram |
Developer Files
| 1 | 1 |
| UCM | Crazy Machines Model File |
Game Files
| 2 | 2 |
| UCN | UltraCompressor II Compressed Archive |
Compressed Files
| 1 | 1 |
| UCS | Relic Localized Unicode Strings |
Game Files
| 3 | 1 |
| UCV | Universal Converter Conversion File |
Data Files
| 1 | 1 |
| UCX | Rational XDE Data |
Data Files
| 3 | 2 |
| UD | OmniPage User Dictionary |
Data Files
| 1 | 1 |
| UDD | OllyDbg Module Information |
Developer Files
| 1 | 1 |
| UDF | Universal Disk Format Image File |
Disk Image Files
| 15 | 6 |
| UDI | Dazzle DVD Complete File |
Uncommon Files
| 2 | 2 |
| UDM | MapView User-Defined Module File |
Game Files
| 1 | 1 |
| UDT | User Defined Type Data |
Data Files
| 1 | 1 |
| UDW | Raw Unsigned DoubleWord (32-bit) Data |
Audio Files
| 1 | 1 |
| UDX | PI Expert Design Software Data |
Data Files
| 2 | 2 |
| UE2 | Encrypted Archive | Encrypted Files | 1 | 1 |
| UEA | Protector Suite QL File | Encoded Files | 1 | 1 |
| UENC | UltraEdit Encrypted Text File |
Encrypted Files
| 1 | 1 |
| UEZ | Ulead COOL 3D Image |
Graphic Files
| 1 | 1 |
| UFD | UFED Physical Analyzer Data |
Data Files
| 1 | 1 |
| UFF | DataWave SciWorks Physiological Data File |
Data Files
| 1 | 1 |
| UFO | Ulead File Object | Raster Image Files | 6 | 4 |
| UFS | FreeBSD Unix File System File |
Disk Image Files
| 2 | 2 |
| UFSD | RAW Recovery Signatures Database File | Database Files | 1 | 1 |
| UG | Drawing File | Vector Image Files | 4 | 1 |
| UGA | Ulead GIF Animator File |
Data Files
| 1 | 1 |
| UGI | MultiGo Game Data |
Game Files
| 1 | 1 |
| UH | Age Of Empires II (The Age Of Kings) File |
Game Files
| 1 | 1 |
| UHS | Universal Hint System Binary Data File |
Uncommon Files
| 1 | 1 |
| UHTM | Unreal Tournament Data File |
Game Files
| 2 | 1 |
| UI | User Interface File |
Developer Files
| 1 | 1 |
| UID | IBM VoiceType User Data File |
Data Files
| 1 | 1 |
| UIF | Universal Image Format Disc Image |
Disk Image Files
| 4 | 1 |
| UIH | Geoworks UI Compiler Espire Header |
Developer Files
| 1 | 1 |
| UIL | AIX User Interface Language |
Data Files
| 1 | 1 |
| UIN | ICQ 2001 Saved ICQ Contact Information |
Data Files
| 1 | 1 |
| UIS | WindowBlinds User Interface Specification |
Settings Files
| 2 | 1 |
| UKX | Unreal Animation Package |
Game Files
| 3 | 1 |
| UK_ | Mcguiness File |
Uncommon Files
| 1 | 1 |
| UL | Windows Updates Downloader Update List File |
Settings Files
| 1 | 1 |
| ULB | COBOL Unisys Library File |
Data Files
| 1 | 1 |
| ULD | Unified Luminaire Data File | CAD Files | 1 | 1 |
| ULF | Universal List File |
Data Files
| 2 | 2 |
| ULG | ULG Bitmap Image |
Raster Image Files
| 1 | 1 |
| ULS | NetMeeting User Location Service File |
Data Files
| 1 | 1 |
| ULT | UltraTracker Module |
Audio Files
| 5 | 1 |
| ULW | U-Law Audio File |
Audio Files
| 2 | 1 |
| ULZ | Windows Updates Downloader Update List File |
Settings Files
| 2 | 2 |
| UM | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) |
Data Files
| 3 | 1 |
| UM4 | Samsung Smart TV Video |
Video Files
| 1 | 1 |
| UMAP | Unreal Engine Map File |
Game Files
| 3 | 1 |
| UMB | MemMaker Backup Archive |
Settings Files
| 1 | 1 |
| UMC | SIMPL Application |
Data Files
| 1 | 1 |
| UMD | Splinter Cell Blacklist Game Data File |
Game Files
| 2 | 2 |
| UMF | MediaHolder Universal Media Format File |
Data Files
| 5 | 2 |
| UMI | Creative PlayCenter CDDB Database File |
Data Files
| 1 | 1 |
| UML | UML Data Object Model |
Developer Files
| 3 | 1 |
| UMLCLASS | UML Class Model |
Developer Files
| 1 | 1 |
| UMOD | Unreal Tournament Module File |
Game Files
| 1 | 1 |
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