Alphabetically: U
Extension File Description File Type
.u
Unreal Tournament Class Package
Game
.u3d
Universal 3D File
3D Image
.u3i
U3 Application Information File
Text
.u3p
U3 Smart Application
Executable
.uax
Unreal Audio Package
Game
.ub
Unsigned Byte Audio File
Audio
.uc
UnrealScript Class
Game
.uc2
UltraCompressor 2 Archive
Compressed
.uccapilog
Microsoft UCC API Log File
Data
.ucf
WebEx Media File
Web
.uci
UFS Explorer Backup File
Backup
.ud
OmniPage User Dictionary
Data
.udb
Dynamics AX User Database File
Database
.udc
Adobe InDesign User Dictionary
Data
.udcx
Universal Data Connection File
Settings
.udeb
Debian Package File
Data
.udf
Universal Disk Format File
Disk Image
.udl
Microsoft Universal Data Link File
Database
.uds
Uniform Data System File
Data
.ufo
Ulead File Object
Raster Image
.ufs
FreeBSD Unix File System File
Disk Image
.ufs.uzip
Compressed Unix File System File
Compressed
.uha
UHarc Compressed Archive
Compressed
.ui
User Interface File
Developer
.uibak
UltraISO Backup Disk Image
Disk Image
.uif
Universal Image Format Disc Image
Disk Image
.uis
WindowBlinds User Interface Specification
Settings
.ukx
Unreal Animation Package
Game
.ulaw
Raw u-Law Audio File
Audio
.ulf
Universal List File
Data
.uls
NetMeeting User Location Service File
Misc
.ult
UltraTracker Module
Audio
.ulw
u-Law Audio File
Audio
.ulz
USBlyzer Data Capture File
Data
.uml
UML Data Object Model
Developer
.umlclass
UML Class Model
Developer
.umod
Unreal Tournament Module File
Game
.umx
Unreal Music Package
Game
.unauth
SiteMinder Unauthorized Message File
Text
.uni
MikMod UniMOD Module
Audio
.unity
Unity Scene File
Game
.unity3d
Unity Web Player File
Game
.unitypackage
Unity Package File
Data
.unk
Unknown File
Misc
.unknown
Unknown File Type
Misc
.unl
Garmin Unlock File
Misc
.unr
Unreal Level Map
Game
.unt
AutoCAD Unit Definition File
CAD
.unx
Unix Text File
Text
.uop
Ultima Online Patch File
Game
.upd
Program Update Information
Text
.update
ISAPI Update File
Data
.upk
Unreal Engine 3 Package
Game
.upoi
iGO Points of Interest File
Data
.upr
FileMaker User Dictionary
Data
.upx
Ultimate Packer for eXecutables File
Executable
.url
Internet Shortcut
Web
.urr
Uniface Signatures File
Misc
.urt
Utah Raster Toolkit File
Raster Image
.usa
Unreal Saved Game File
Game
.user
Visual Studio Project User Options File
Settings
.userprofile
Norton Antivirus User Profile File
Settings
.usf
Nintendo 64 Music File
Audio
.usflib
Nintendo 64 Song Library
Audio
.usr
FileMaker Pro Database File
Database
.ustar
Uniform Standard Tape Archive Format File
Data
.usx
Unreal Static Meshes
Game
.ut2
Unreal Tournament Map
Game
.ut2mod
Unreal Tournament 2003/2004 Module File
Game
.ut3
Unreal Tournament 3 Map File
Game
.ut4mod
Unreal Tournament 2004 Module
Game
.utc
Bioware Creature File
Game
.utf8
Unicode UTF8-Encoded Text Document
Text
.utw
Unity Whirld File
Game
.utx
Unreal Texture Package
Game
.utxt
Unicode Text File
Text
.utz
UIQ Theme Package
Settings
.uu
Uuencoded File
Encoded
.uud
UUDecoded File
Encoded
.uue
Uuencoded File
Encoded
.uvf
Uncertain Value Format
Data
.uvx
Unreal Tournament 2003 Saved Game
Game
.uw
Unsigned Word Audio File
Audio
.uwf
UltraTracker Wave File
Audio
.uwl
User Word List File
Data
.uxx
Unreal Tournament Cache
Game
.uzip
FreeBSD Compressed Disk Image
Compressed