| una | SIMS Unauthorized Data |
| unauth | CA SiteMinder Unauthorized Message |
| und | MOUSE Urban Network Datafile |
| unk | Lotus Notes Unknown Attachment File |
| unl | Garmin Unlock Key |
| unrst | OPM Flow Data |
| unt | EES Units List |
| unt | Autodesk AutoCAD Unit Definition |
| unv | I-DEAS UNV File |
| upd | Program Update Information |
| upf | Unified Power Format |
| upl | CYANA Upper Distance Limit File |
| upm | PenMail Data |
| upoi | iGO 2008 POI Database |
| upr | FileMaker Pro User Dictionary |
| ups | VisualBoy Advance UPS Patch File |
| upt | Connectivity Memory Model Update Timing File |
| upz | Starmoney Agent Download Data |
| urb | BlueM Consumers Data |
| urd | Ultra Recall Database |
| uri | Uniform Resource Identifiers 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 |
| usm | Epson PIF Tool Multiple Size Frame |
| usr | Pegasus Mail User Data |
| usr | NEK5000 USRfile |
| uss | WinUAE Saved State |
