| 1 | U | Epic Games | Unreal Tournament Class Package |
| 2 | U01 | Barudan | Barudan Embroidery |
| 3 | U11 | Dr Tax Software | UFile 2011 Tax Return |
| 4 | U12 | Dr Tax Software | UFile 2012 Tax Return |
| 5 | U3D | 3D Industry Forum | Universal 3D Format |
|
| 6 | U3P | SanDisk | U3 Smart Application |
| 7 | UAQ | Microsoft Corporation | Microsoft Windows Action Queue |
| 8 | UASSET | Epic Games | Unreal Asset |
| 9 | UAX | Epic Games | Unreal Audio Package |
| 10 | UC | N/A | UnrealScript Class |
| 11 | UCCAPILOG | Microsoft | Microsoft UCC API Log Format |
| 12 | UCE | Microsoft | Windows UCE System Format |
| 13 | UCF | Cisco Systems | WebEx Media Format |
| 14 | UCL | Epic Games, Inc. | Unreal Tournament Cache Format |
| 15 | UCS | Relic Entertainment Inc. | Homeworld Localized Unicode Strings |
| 16 | UDC | Adobe Systems | Adobe InDesign User Dictionary |
| 17 | UDCX | Microsoft | Universal Data Connection Format |
| 18 | UDF | Optical Storage Technology Association | Universal Disk Format |
| 19 | UDIF | Apple, Inc. | Apple Mac OS X Universal Disk Image Format |
| 20 | UDK | Epic Games | Unreal Engine Map Format |
| 21 | UDL | Microsoft | Microsoft Universal Data Link Format |
| 22 | UDT | N/A | Generic |
| 23 | UFS | The FreeBSD Project | FreeBSD Unix Format System Format |
| 24 | UFX | Ultimate 3D | Ultimate 3D Game Engine Material |
| 25 | UHA | UHarc | UHarc Compressed Archive |
| 26 | UHTM | Epic Games, Inc. | Unreal Tournament |
| 27 | UI | Nokia | User Interface Format |
| 28 | UIF | MagicISO | Universal Image Format Disc Image |
| 29 | UIS | Stardock | WindowBlinds User Interface Specification |
| 30 | UIX | Oracle Corporation | UiXML Page |
| 31 | UK | N/A | English (British) Language Translation |
| 32 | UKR | N/A | Adobe Acrobat language resource data |
| 33 | UKX | Epic Games | Unreal Animation Package |
| 34 | ULT | N/A | UltraTracker Module |
| 35 | UMAP | Epic Games | Unreal Engine Map File |
| 36 | UMF | GetMediaHolder | MediaHolder Universal Media Format |
| 37 | UML | N/A | UML Data Object Model |
| 38 | UMLX | Visual Object Modelers, Inc. | Visual UML XML Model |
| 39 | UMOD | Epic Games | Unreal Tournament Module Format |
| 40 | UNR | N/A | Unreal Level Map |
| 41 | UNV | SAP AG | BusinessObjects Universe |
| 42 | UNX | N/A | Unix Text Format |
| 43 | UOS | N/A | Uniform Office Spreadsheet |
| 44 | UOT | N/A | Uniform Office Document |
| 45 | UPD | N/A | Program Update Information |
| 46 | UPI | N/A | Ulead PhotoImpact Plugin |
| 47 | UPJ | N/A | Ulead Project |
| 48 | UPK | Epic Games | Unreal Engine 3 Package |
| 49 | UPOI | Nav N Go | IGO Points Of Interest Format |
| 50 | URL | N/A | Internet Shortcut |
| 51 | URL_ | N/A | Internet Shortcut - URL - Uniform Resource Locator |
| 52 | US | N/A | English Language Translation |
| 53 | USER | Microsoft | Visual Studio Project User Options Format |
| 54 | USERKIT | N/A | DrumCore audio |
| 55 | USERSETTINGS | N/A | User settings |
| 56 | USF | EVGA | Nintendo 64 Music Format |
| 57 | USM | N/A | USM Game Video Format |
| 58 | USR | FileMaker | FileMaker Pro Database Format |
| 59 | USTAR | N/A | Uniform Standard Tape Archive Format |
| 60 | USX | Epic Games | Unreal Static Meshes |
| 61 | UT2 | Epic Games | Unreal Tournament Map |
| 62 | UT2MOD | Epic Games | Unreal Tournament 2003/2004 Module Format |
| 63 | UT3 | Epic Games | Unreal Tournament 3 Map Format |
| 64 | UT4MOD | Epic Games | Unreal Tournament 2004 Module |
| 65 | UTF8 | N/A | Unicode UTF8-Encoded Text Document |
| 66 | UTX | Epic Games | Unreal Texture Package |
| 67 | UU | N/A | Uuencoded Format |
| 68 | UUE | N/A | Uuencoded Format |
| 69 | UVR | N/A | UVR Mesh |
| 70 | UXDC | Microsoft Corporation | Microsoft Excel Data Retrieval Service Connection |
| 71 | UXY | Barudan | Barudan Embroidery Outline |
| 72 | UYVY | N/A | YUV 16-bit Image Format |
| 73 | UZ2 | Epic Games, Inc. | Unreal Tournament Compressed |
| 74 | UZY | N/A | Backdoor.Sokacaps |
