| U | Subsampled Raw YUV Image |
| UAP | User agent profile (Used by wireless telephony applications) |
| UB | Raw unsigned byte (8-bit) data |
| UC2 | Compressed archive (UltraCompressor II) |
| UCN | New compressed archive (UltraCompressor II) |
| UDB | Off-line application file used with Rockwell’s Ultraware software |
| UDF | Image filter (Photostyler) |
| UDW | Raw unsigned doubleword (32-bit) data |
| UE2 | Encrypted archive (UltraCompressor II) |
| UFO | Object file (Ulead) |
| UG | Drawing file (AutoCAD and others) |
| UHS | Binary file (Universal Hint System) |
| UI | User interface file (Sprint) |
| UIF | Long prompts for Microsoft Windows (WordPerfect for Windows) |
| UIH | Espire header file (Geoworks UI Compiler) |
| UL | Audio file (ULAW) |
| ULAW | (CCITT G.711) audio (US Telephony) |
| ULD | Uploaded file information (Procomm Plus) |
| ULT | Music module (MOD) (UltraTracker) |
| UMB | Backup file (archive) (MemMaker) |
| UNC | Files and Settings Transfer Wizard Output Folder (Note: folder format) |
| UNI | Datafile (Forcast Pro) |
| UNX | Text file (Unix specific information) |
| UPD | Updated program data (dBase) |
| UPI | Program file (ULead Photo Impact) |
| UPO | Compiled updated datafile (dBase) |
| UPX | Saved Image file (ULead Photo Express) |
| URF | Universal radar format (Radar ViewPoint) |
| URL | Internet shortcut file (Universal Resource Locator) |
| USA | Unreal II Savegame File |
| USE | Source Integrity file (MKS) |
| USP | Printer font w/updated USACII extended character set (Pagemaker) |
| USR | Audit trail file (Pro/Engineer) |
| UU | Compressed ASCII archive (UUDE/ENCODE) |
| UUE | UU-encoded file |
| UW | Raw unsigned 16-bit) datafile (Word) |
| UWF | Wave file (UltraTracker) |
