| xsw | Milnta APL Transfer Function Data |
| xsy | Unity Pro Exported Variables |
| xsym | CodeWarrior xSYM File |
| xtml | XML4web HTML Document |
| xul | XML User Interface Language File |
| y | Yacc Source |
| yab | Yabasic Source Code |
| yac | Yacc Source |
| yacc | Yacc Source |
| yajl | YAJL Script |
| yaws | Yaws Server Dynamic Page |
| ygl | YAGL Script |
| ymx | Yukes 3D Object |
| yo | Y86 Assembler Object |
| yobj | Yukes 3D Object |
| ypp | GNU Bison Source Code |
| ys | Y86 Assembler Source Code |
| ysp | BYOB Sprite |
| yxx | GNU Bison Source Code |
| yy | GNU Bison Parser |
| zasm | Zero Compiler Assembly Source Code |
| zbj | Visual Zero Object |
| zbl | Zeble Markup Data |
| zcls | Zero Compiler Class Data |
| zcos | Scilab XML Diagram |
| zef | Unity Pro Full Application Exchange Data |
| zem | Zope External Editor File |
| zep | Zephir Source Code |
| zepto | Zepto JS Script |
| zexp | Zope Binary Export File |
| zf3d | Flare3D Model |
| zfd | ABC Zeroadic Functions Data |
| zfrm | Zero Form Designer Data |
| zh | Netscape Navigator Java Classes File |
| zodb | Zope Object Database |