| .U | Unreal Tournament Class Package | 234 |
| .U | AU Audio File | 267 |
| .U | Subsampled Raw YUV Image | 367 |
| .U10 | UFile 2010 Tax Return | 200 |
| .U11 | UFile 2011 Tax Return | 200 |
| .U12 | UFile 2012 Tax Return | 300 |
| .U3D | Universal 3D File | 273 |
| .U3I | U3 Application Information File | 267 |
| .U3P | U3 Smart Application | 220 |
| .U8 | Mario Kart Wii Archive | 200 |
| .U8 | 3D Lemmings Audio File | 217 |
| .UASSET | Unreal Asset File | 179 |
| .UAX | Unreal Audio Package | 229 |
| .UB | Unsigned Byte Audio File | 400 |
| .UBJ | Universal Binary JSON File | 275 |
| .UBOX | Universe Sandbox Data File | 271 |
| .UC | UnrealScript Class | 222 |
| .UC2 | UltraCompressor 2 Archive | 300 |
| .UCCAPILOG | Microsoft UCC API Log File | 257 |
| .UCE | Windows UCE System File | 300 |
| .UCF | WebEx Media File | 270 |
| .UCI | UFS Explorer Backup File | 300 |
| .UCL | Killing Floor System File | 300 |
| .UCM | Crazy Machines Model File | 500 |
| .UCT | UC Browser Theme File | 200 |
| .UD | OmniPage User Dictionary | 400 |
| .UDB | Dynamics AX User Database File | 270 |
| .UDC | Adobe InDesign User Dictionary | 400 |
| .UDCX | Universal Data Connection File | 175 |
| .UDEB | Debian Package File | 400 |
| .UDF | Universal Disk Format File | 287 |
| .UDF | Excel User Defined Function | 322 |
| .UDK | Unreal Engine Map File | 237 |
| .UDL | Microsoft Universal Data Link File | 200 |
| .UDS | Uniform Data System File | 500 |
| .UDS | Sierra Generations File | 380 |
| .UDS | NetBackup UNIX Domain Socket File | 367 |
| .UEA | Protector Suite QL Encrypted Archive | 400 |
| .UFO | Unified Font Object File | 400 |
| .UFO | Ulead File Object | 267 |
| .UFR | Upfiring File | 400 |
| .UFR | Ultra Fractal File | 350 |
| .UFS | FreeBSD Unix File System File | 475 |
| .UFS.UZIP | Compressed Unix File System File | 250 |
| .UFT | Torque Font Cache File | 275 |
| .UGA | Ulead GIF Animator File | 350 |
| .UGOIRA | Ugoira Animated Image File | 325 |
| .UGR | Apophysis Ultra Fractal Gradient File | 275 |
| .UHA | UHarc Compressed Archive | 197 |
| .UHTML | UC Browser Saved Webpage | 241 |
| .UI | User Interface File | 167 |
| .UIBAK | UltraISO Backup Disk Image | 220 |
| .UIF | Universal Image Format Disc Image | 252 |
| .UIS | WindowBlinds User Interface Specification | 200 |
| .UKX | Unreal Animation Package | 257 |
| .ULAW | Raw u-Law Audio File | 300 |
| .ULD | Unified Luminaire Data File | 250 |
| .ULF | Universal List File | 300 |
| .ULP | EAGLE User Language Program | 300 |
| .ULS | NetMeeting User Location Service File | 333 |
| .ULT | UltraTracker Module | 350 |
| .ULW | u-Law Audio File | 450 |
| .ULX | Glulx Game File | 467 |
| .ULZ | USBlyzer Data Capture File | 380 |
| .UMAP | Unreal Engine Map File | 210 |
| .UMD | Splinter Cell Blacklist Game Data File | 267 |
| .UMF | MediaHolder Universal Media Format File | 400 |
| .UML | UML Data Object Model | 277 |
| .UMLCLASS | UML Class Model | 400 |
| .UMOD | Unreal Tournament Module File | 333 |
| .UMS | Rune 3D Mesh File | 300 |
| .UMX | Unreal Music Package | 145 |
| .UNAUTH | SiteMinder Unauthorized Message File | 400 |
| .UNF | UNIF ROM File | 275 |
| .UNI | MikMod UniMOD Module | 220 |
| .UNIF | UNIF ROM File | 400 |
| .UNITY | Unity Scene File | 161 |
| .UNITY3D | Unity Web Player Archive | 156 |
| .UNITYPACKAGE | Unity Package File | 177 |
| .UNITYPROJ | Unity 3D Project | 243 |
| .UNK | Unknown File | 392 |
| .UNKNOWN | Unknown File Type | 193 |
| .UNL | Garmin Unlock File | 236 |
| .UNR | Unreal Level Map | 183 |
| .UNT | AutoCAD Unit Definition File | 400 |
| .UNX | Unix Text File | 400 |
| .UOF | Uniform Office Document | 400 |
| .UOP | Uniform Office Presentation | 400 |
| .UOP | Ultima Online Patch File | 400 |
| .UOS | Uniform Office Spreadsheet | 350 |
| .UOT | Uniform Office Document | 333 |
| .UPD | Program Update Information | 213 |
| .UPDATE | ISAPI Update File | 182 |
| .UPDATR | iConomy Update File | 286 |
| .UPF | Panono Unstitched Panorama Format | 400 |
| .UPF | MicroStation User Preferences File | 289 |
| .UPF | Unified Power Format File | 450 |
| .UPG | Upgrade File | 307 |
| .UPK | Unreal Engine 3 Package | 167 |
| .UPOI | iGO Points of Interest File | 250 |
| .UPR | FileMaker User Dictionary | 450 |
| .UPR | Fractal Parameters File | 400 |
| .UPS | UPS Patch File | 284 |
| .UPX | Ultimate Packer for eXecutables File | 333 |
| .UQM | The Ur-Quan Masters Game Data File | 500 |
| .URL | Internet Shortcut | 212 |
| .URR | Uniface Signatures File | 500 |
| .URT | Utah Raster Toolkit File | 500 |
| .USA | Unreal Saved Game File | 217 |
| .USD | Universal Scene Description Format | 300 |
| .USDZ | Universal Scene Description Zipped Format | 400 |
| .USEQ | USeq Genome Data File | 400 |
| .USER | Visual Studio Project User Options File | 206 |
| .USERPROFILE | Norton AntiVirus User Profile File | 267 |
| .USERTILE-MS | Windows 8 User Tile File | 200 |
| .USF | Universal Subtitles Format File | 350 |
| .USF | Nintendo 64 Music File | 320 |
| .USF | EVGA Precision X Skin File | 287 |
| .USFLIB | Nintendo 64 Song Library | 350 |
| .USM | USM Game Video File | 232 |
| .USR | Lowrance GPS Data File | 283 |
| .USR | SmartMusic Custom File | 207 |
| .USR | FileMaker Pro Database File | 222 |
| .UST | UTAU Sequence Text File | 190 |
| .USTAR | Uniform Standard Tape Archive Format File | 500 |
| .USX | Unreal Static Meshes | 150 |
| .UT2 | Unreal Tournament Map | 433 |
| .UT2MOD | Unreal Tournament 2003/2004 Module File | 333 |
| .UT3 | Unreal Tournament 3 Map File | 275 |
| .UT4MOD | Unreal Tournament 2004 Module | 200 |
| .UT8 | Ghost Master Language File | 300 |
| .UTC | Bioware Creature File | 280 |
| .UTF8 | Unicode UTF8-Encoded Text Document | 278 |
| .UTW | Unity Whirld File | 400 |
| .UTX | Unreal Texture Package | 193 |
| .UTXT | Unicode Text File | 400 |
| .UTZ | UIQ Theme Package | 250 |
| .UU | Uuencoded File | 233 |
| .UUD | UUDecoded File | 333 |
| .UUE | Uuencoded File | 229 |
| .UVF | Uncertain Value Format | 200 |
| .UVW | UVW Coordinates File | 233 |
| .UVX | Unreal Tournament 2003 Saved Game | 433 |
| .UW | Unsigned Word Audio File | 225 |
| .UWF | UltraTracker Wave File | 275 |
| .UWL | User Word List File | 200 |
| .UWRF | Universal Web Radio File | 250 |
| .UXF | UML eXchange Format | 220 |
| .UXX | Unreal Tournament Cache | 241 |
| .UZIP | FreeBSD Compressed Disk Image | 350 |