| xvs | XView Scene Package |
| xvu | Comodo BOClean Definitions |
| xvv | Placevent 3D-RISM Results Data |
| xwz | X-Setup Pro Wizard Data |
| xx | Xxencoded Data |
| xxe | Xxencoded Data |
| xxi | GameSpy Arcade Data |
| xxl | Xilog Plus XXL File |
| xxx | Singer XL Embroidery Data |
| xya | ANUGA XYA Data |
| xye | TOPAS Data |
| xyh | TunnelCAD Coordinate Data |
| xys | XYplorer Script |
| xyz | Tinker Cartesian Coordinates |
| xyz | Celestia Trajectory File |
| xyzv | Celestia v1.6 Trajectory File |
| xzp | Freestyle Dash Skin |
| x_b | Parasolid CAD Binary Parts Data |
| x_t | Parasolid CAD ASCII Parts Data |
| y01 | Corel Paradox Secondary Index |
| y02 | Corel Paradox Secondary Index |
| y03 | Corel Paradox Secondary Index |
| y04 | Corel Paradox Secondary Index |
| y05 | Corel Paradox Secondary Index |
| y06 | Corel Paradox Secondary Index |
| y07 | Corel Paradox Secondary Index |
| y08 | Corel Paradox Secondary Index |
| y09 | Corel Paradox Secondary Index |
| yaml | YAML Document |
| yarn | Yarn Data |
| ycd | Y-Cruncher Data |
| ycf | FKR Program YCF Data |
| ychat | Yahoo! Messenger Chat Log |
| ycr | YHC Core |
| yct | YahELite Chat Transcript Data |
