| Extension | Description | File Type |
|---|
| u | Unreal Tournament File | 3D Files |
| uccapilog | UCC API Log File | Data Files |
| udstate | Android Media Finder File | System Files |
| ufo | PhotoImpact Graphic File | Graphic Files |
| uha | UHARC compressed archive File | Archived Files |
| uibak | Backup Disk Image File | Disk Image Files |
| uif | MagicISO Disc Image File | Disk Image Files |
| ukx | Unreal Tournament Animation File | Game Files |
| umd | Universal Media Disc File | Disk Image Files |
| unknown | Unknown File | Misc Files |
| unl | Garmin Unlock File | Text Files |
| uns2 | Ultra Notes Backup file | Backup Files |
| upd | Update File | System Files |
| update | ISAPI Update File | System Files |
| upg | Firmware Upgrade File | Misc Files |
| upk | Mass Effect Game File | Game Files |
| url | Uniform Resource Locator File | Web Files |
| urls | GetRight Url List File | Misc Files |
| user | Visual Studio User Options File | Settings Files |
| usr | User Database File | Settings Files |
| utx | Unreal Engine Texture File | Graphic Files |
| uwl | User Word List File | Misc Files |
| uwl | User Word List File | Misc Files |