| yob | YASARA Object |
| ypf | YahElite Personality File |
| ypm | SigmaWinLite Data |
| yrcbck | YRC Weblink Backup Archive |
| yrcbkm | YRC Weblink Bookmarks Archive |
| yrcdat | YRC Weblink Data Script |
| yro | WRAP Yield Versus Reliability Output Data |
| yse | Yamaha Studio Manager Session Data |
| yss | YcySoft Screen Saver Producer Data |
| ytdl | youtube-dl Downloaded Fragment |
| ytf | YTGen Population Data |
| ytm | YTGen Method Data |
| ytp | madVR Test Pattern |
| ytr | IRIS OCR Data |
| yum | YUM Data |
| yumtx | YUM Transaction Data |
| yves | YVES Bible Plugin Data |
| ywl | YoWindow Landscape Data |
| yxbe | Alteryx Designer Browse Everywhere Data |
| yxdb | Alteryx Designer Database |
| yxft | Alteryx Designer Field Types Data |
| yxi | Alteryx Designer Packaged Tool Data |
| yxlang | Alteryx Designer Localization Data |
| yxlc | Alteryx Designer License Data |
| yxmc | Alteryx Designer Macro Data |
| yxmd | Alteryx Designer Workflow Data |
| yxwg | Alteryx Designer Workflow Group Data |
| yxwv | Alteryx Designer App Values Data |
| yxzp | Alteryx Designer Packaged Workflow Data |
| z1 | Z-machine Z-code File |
| z2 | Z-machine Z-code File |
| z3 | Z-machine Z-code File |
| z32 | ZAR32 Saved Progress Data |
| z3l | ZW3D Part Library |
| z4 | Z-machine Z-code File |