| 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 |
| yef | Yoda Export Data |
| yg0 | Corel Paradox Secondary Index |
| yg1 | Corel Paradox Secondary Index |
| yg2 | Corel Paradox Secondary Index |
| yg3 | Corel Paradox Secondary Index |
| yg4 | Corel Paradox Secondary Index |
| ygo | YGOPRO Data |
| yld | SMS Yield Monitor Data |
| yls | YuleLog Data |
| ymg | Yahoo! Messenger Data |
| yml | YAML Document |
| ymn | Yellicode Modeler Model Data |
| ymt | Yar Material File |
| yob | YASARA Object |
| ypf | YahElite Personality File |
| yrcbck | YRC Weblink Backup Archive |
| yrcbkm | YRC Weblink Bookmarks Archive |
| yrcdat | YRC Weblink Data Script |
| yse | Yamaha Studio Manager Session Data |