File extensions starting with the letter U...

  .U    Unreal System File
  .U96  EasyZip Temporary File
  .UAP  User Agent Wireless Telephony Profile
  .UAX  Unreal Audio File
  .UB   Audio File
        Raw Unsigned Byte (8-bit) Data
  .UC2  Compressed File  
  .UCM  Urban Chaos Game File
  .UCN  Compressed Archive 
  .UDF  WindowsNT Uniqueness Database File
        Photostyler Image Filter
  .UDL  MS Data Link
  .UDW  Raw Unsigned DoubleWord (32-bit) Data
  .UE2  Encrypted Archive 
  .UFO  Ulead Object File
  .UG   Drawing File (various)  
  .UHS  Universal Hint System File
  .UI   User Interface
        Geoworks UI Compiler Espire Source
  .UIF  WordPerfect Long Prompts
  .UIH  Geoworks UI Compiler Espire Header
  .UL   uLAW Audio File
  .ULD  Procomm Uploaded Files
  .ULS  Internet Location Service
  .ULT  UltraTracker Music Module
  .UMB  MemMaker Backup Archive
  .UMOD  Unreal Self-extracting MOD File
  .UMX  Unreal Music File
  .UNI  MikMod (UniMod) Format
        Forcast Pro Data File
  .UNR  Unreal Map File
  .UNX  Text File (usually UNIX)
  .UPD  dBase Update File
        Universal Print Driver (cross-platform)
        Updated Program Data (various)
  .UPI  ULead Photo Impact Program File
  .UPJ  ULead Project File
  .UPO  dBase Compiled Update File
  .UPX  ULead Photo Express Saved Image File
  .URF  Radar ViewPoint Universal Radar Format
  .URL  Internet Location
  .USE  MKS Source Integrity File
  .USL  LaserJet Landscape Font
  .USP  PageMaker Printer Font
        LaserJet Portrait Font
  .USR  User Database (various)
  .UTF  AOL Updating Files (found in TOD [Tools on Demand] folder)
  .UTL  Sound File
  .UTP  BinPatch Patch Data File
  .UTX  Unreal Texture File
  .UU   Compressed Archive File (UUENCODE)
  .UUD  Uudecoded
  .UUE  Uuencoded
  .UVR  Ulead Cool 360 Viewer File
  .UW   Audio File
        Raw Unsigned Word (16-bit) Data
        Unitype GlobalWriter File
  .UWF  UltraTracker Wave File