| Y | Compressed Amiga archive (YABBA) |
| Y01 | Secondary index file (Paradox) |
| Y02 | Secondary index file (Paradox) |
| Y03 | Secondary index file (Paradox) |
| Y04 | Secondary index file (Paradox) |
| Y05 | Secondary index file (Paradox) |
| Y06 | Secondary index file (Paradox) |
| Y07 | Secondary index file (Paradox) |
| Y08 | Secondary index file (Paradox) |
| Y09 | Secondary index file (Paradox) |
| YBK | Yearbook file (Microsoft Encarta) |
| YUV | Graphics file (YUV) |
| YZ | Compressed file archive (YAC) |
