| upt | Connectivity Memory Model Update Timing File |
| upvc | UniversalPlantViewer v4 Model Data |
| upw | MODFLOW Upstream Weighting Package |
| upz | Starmoney Agent Download Data |
| urb | BlueM Consumers Data |
| urd | Ultra Recall Database |
| uri | Uniform Resource Identifiers List |
| urls | Getright Internet Shortcuts List |
| urn | H3DAPI Uniform Resource Name Data |
| urq | Review QuickChart Chart Data |
| urrs | UFS Explorer IntelliRAW Rules |
| urs | ControlDraw User Requirements Specifier |
| usb | SiSoftware Sandra USB Data |
| usd | Epson Print Image Framer Tool Standard Frame |
| usdz | ARKit USDZ Augmented Reality Data |
| use | MKS Source Integrity Project Cache |
| useq | USeq Genome Data |
| user | Unix Filesystem User Quota Data |
| usk | Ultraplayer Playlist Data |
| usm | Epson PIF Tool Multiple Size Frame |
| usr | NEK5000 USRfile |
| usr | Pegasus Mail User Data |
| usr | SigmaWin+ User Level Data |
| uss | WinUAE Saved State |
| usz | Stardock SkinStudio Data |
| utf | AOL Software Update |
| utg | LeCroy USBTracer Generation Data |
| uu | Uuencoded Data |
| uue | Uuencoded Data |
| uuid | Linux UUID Data |
| uum | Minuum Keyboard Backup |
| uus | Ultra Utility Skin Data |
| uva | BRE U-value Calculator Data |
| uvf | Netica Uncertain Value Format File |
| uvl | µVision Signal Definition |