File extensions start with character 'U'
| File Type | Category | File Format | |
|---|---|---|---|
| .u3i | Text Files | U3 Application Information File | |
| .ubj | Data Files | Universal Binary JSON File | |
| .uccapilog | Microsoft Files | Microsoft UCC API Log File | |
| .ud | Data Files | OmniPage User Dictionary | |
| .udc | Data Files | Adobe InDesign User Dictionary | |
| .udeb | Data Files | Debian Package File | |
| .uds | Data Files | Sierra Generations File | |
| .uea | Data Files | Protector Suite QL Encrypted Archive | |
| .uha | Archive Files | UHarc Compressed Archive | |
| .uhtml | HTML Files | UC Browser Saved Webpage | |
| .uibak | Backup Files | UltraISO Backup Disk Image | |
| .uif | Disk Image Files | Universal Image Format Disc Image | |
| .ulf | Data Files | Universal List File | |
| .ulz | Data Files | USBlyzer Data Capture File | |
| .umf | Data Files | MediaHolder Universal Media Format File | |
| .unauth | Text Files | SiteMinder Unauthorized Message File | |
| .unity3d | Game Files | Unity Web Player File | |
| .unitypackage | Package Files | Unity Package File | |
| .unl | Multiple Use Files | Garmin Unlock File | |
| .unx | Text Files | Unix Text File | |
| .uoml | Text Files | Unstructed Operation Markup Language File | |
| .upd | Text Files | Program Update Information | |
| .update | Data Files | ISAPI Update File | |
| .upg | Multiple Use Files | Upgrade File | |
| .upk | Game Files | Unreal Engine 3 Package | |
| .upr | Data Files | Fractal Parameters File | |
| .url | Web Files | Internet Shortcut | |
| .usm | Digital Video Files | USM Game Video File | |
| .ustar | Data Files | Uniform Standard Tape Archive Format File | |
| .utf8 | Text Files | Unicode UTF8-Encoded Text Document | |
| .utxt | Text Files | Unicode Text File | |
| .uvf | Data Files | Uncertain Value Format | |
| .uwl | Data Files | User Word List File | |
| .uzip | Disk Image Files | FreeBSD Compressed Disk Image |