| ugrid | Delft3D Flexible Mesh Suite UGRID Data |
| ugz | Compressed Go Game Data |
| uhh | Eurotherm Hydra History Data |
| uhs | Universal Hint System Hints File |
| uid | IBM VoiceType User Data |
| uid-zps | Zoner Photo Studio Changes Backup Data |
| uil | AIX User Interface Language |
| uin | ICQ 2001 Saved ICQ Contacts Information |
| uis | Stardock WindowsBlinds Style Description |
| uiz | SpyBot Search and Destroy Updater Data |
| uke | Ulead VideoStudio Data |
| ukr | Adobe Acrobat Reader Ukrainian Language Data |
| uky | UltraMixer Licence Key Data |
| uld | DIALux Unified Luminaire Data |
| ulf | UMX Language File |
| ulog | PX4 ULog Data |
| ulx | Glulx Game File |
| ulz | USBlyzer Captured Data |
| um | The Land of UM Data |
| umf | Media Holder Universal Media Format |
| umg | USoft User Message Data |
| umlx | Visual UML XML Model Data |
| umm | Unified Molecular Modeling Data |
| umr | MyEclipse UML Model Repository Data |
| una | SIMS Unauthorized Data |
| unauth | CA SiteMinder Unauthorized Message |
| und | MOUSE Urban Network Datafile |
| unityweb | Unity Webplayer Data |
| unixyz | UniChem Chemical Modeller XYZ Data |
| unk | Lotus Notes Unknown Attachment File |
| unl | Garmin Unlock Key |
| unrst | OPM Flow Data |
| unt | Autodesk AutoCAD Unit Definition |
| unt | EES Units List |
| unv | I-DEAS UNV File |