| ugrid | Delft3D Flexible Mesh Suite UGRID Data |
| ugz | Compressed Go Game Data |
| uhh | Eurotherm 6100a Binary Data |
| uhh | Eurotherm Hydra History Data |
| uhi | Eurotherm 6100a Printer Driver Import Data |
| uhq | Eurotherm 6100a Support Data |
| uhr | Eurotherm 6100a User Resource Data |
| uhs | Universal Hint System Hints File |
| uht | Eurotherm 6100a Table Data |
| uhu | Eurotherm 6100a User Screen Data |
| 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 |
| ukm | UltraMixer Key Mapping Data |
| ukr | Adobe Acrobat Reader Ukrainian Language Data |
| uky | UltraMixer Licence Key Data |
| uld | DIALux Unified Luminaire Data |
| ulf | UMX Language File |
| uli | Microsoft Office ULI Data |
| ulog | PX4 ULog Data |
| ulx | Glulx Game File |
| ulz | USBlyzer Captured Data |
| um | The Land of UM Data |
| um3 | eXtendable Wad Editor UM3 Data |
| umf | Media Holder Universal Media Format |
| umg | USoft User Message Data |
