File Extensions beginning with the letter 'U'

Extension	Description	Format	MIME Type
U3D	Universal 3D. A vector image format for 3D images. More info.
UB	Raw unsigned byte (8-bit) data
UC2	Archive; Ultra-Compressor II
UCN	Archive; Ultra-Compressor II
UDF	Universal Disk Format. A disk image, similar to .ISO
UDF	Uniqueness Database File. Used for automated installation on Microsoft Windows NT and later. Details.
UDW	Raw unsigned doubleword (32-bit) data
UE2	Encrypted Archive; Ultra-Compressor II
UFO	Unified Font Object. Details
uid
UL	Sound; Ulaw
ULAW	US telephony format (CCITT G.711) audio
ULS	Internet; User Location Service (Internet New Media Phone) (MIME: text/iuls)
ULT	Sound; Ultra-Tracker Music
ULT	Ultratracker music module (MOD) file
UNI	MikMod UniMod formatted file
UNI	Data; Forecast-Pro
UPD	Text Document (update)
URL	Internet shortcut file
url	uniform resource location
URL	Internet Shortcut
URL	Internet Shortcut
URT	Utah Raster Toolkit
USE	MKS Source Integrity file
USL	Font; Hewlett-Packard Laser Printer Landscape Font
USP	Font; Hewlett-Packard Laser Printer Portrait Font
USR
UTD	Universal Text Data. Details
UTL	QuickBooks; Data
UU	UU-encoded file
UU	Archive; Not necessarily compressed, may be UUE
uue	Uuencoded file (encoded ascii)
UUE	UU-encoded file
UUE	Archive; UUE Encoded Archive; see XXE
uue	UUEncoded
UW	Sound
UW	Raw unsigned word (16-bit) data
UWF	UltraTracker Wave file
UWL