| Extension | Information |
| Y | Amiga Yabba Compressed Archive (Amiga) |
| Y | Desktop Color Separation Specification Yellow Layer |
| Y | Yacc: Yet Another Compiler-Compiler Grammar File |
| Y01 | Paradox Secondary Index |
| Y02 | Paradox Secondary Index |
| Y03 | Paradox Secondary Index |
| Y04 | Paradox Secondary Index |
| Y05 | Paradox Secondary Index |
| Y06 | Paradox Secondary Index |
| Y07 | Paradox Secondary Index |
| Y08 | Paradox Secondary Index |
| Y09 | Paradox Secondary Index |
| Y2 | Plugh! |
| Y64 | Commandos 2 Game Mission |
| YAB | YaBasic Source |
| YAL | Arts & Letters Clipart |
| YBHTM | yBook HTML File (Spacejock Software) |
| YBK | Encarta Yearbook |
| YBM | Face Bitmap |
| YBS | YBS Compressed Archive |
| YBTXT | yBook Text File (Spacejock Software) |
| YC | YAC Compressed Archive |
| YDC | Yu-Gi-Oh! Power of Chaos Card Deck (Konami Group) |
| YDK | Yusuf Data Recovery |
| YEL | Yellow Color Separation (Adobe) |
| YENC | yEnc Encoded File |
| YG0 | Paradox Secondary Index |
| YG1 | Paradox Secondary Index |
| YG2 | Paradox Secondary Index |
| YG3 | Paradox Secondary Index |
| YG4 | Paradox Secondary Index |
| YGM | YahooGroupManager Ver. 1.x Database (GGNetworks) |
| YGM2 | YahooGroupManager Ver. 2.x Database (GGNetworks) |
| YGN | yGen Project File (Spacejock Software) |
| YGO | Paradox Database-related File |
| YIF | Graphic |
| YLD | SMS Yield Monitor Data File (Ag Leader Technology, Inc.) |
| YLT | TheWord & Bible Companion Young's Literal Translation Bible |
| YM | Atari ST YM2149 Module/Song (Atari) |
| YMACRO | YaHoE File |
| YMAP | Ygnius Computer Aided Thinking File |
| YMG | Yahoo! Messenger (Yahoo!) |
| YML | Sailfish Yacht Analyzer XML-based Data File (Techsail) |
| YML | Ruby on Rails Database Configuration File (Ruby on Rails Project) |
| YMP | MicroSim PCBoard Mapping File |
| YN | Audio Utility Tuningwrench Compressed File |
| YNC | yEnc Encoded File |
| YNX | Seqmax Seqprest Compressed Audio File |
| YPK | MicroSim PCBoard Mapping File |
| YPL | Yahoo! Player Playlist |
| YPS | Yahoo! Messenger (Yahoo!) |
| YPT | Cryptic File |
| YRM | Command & Conquer Yuri's Revenge Unofficial Multiplayer Map (Westwood Studios) |
| YRO | Command & Conquer Yuri's Revenge Official Multiplayer Map (Westwood Studios) |
| YSP | Bitmap Graphic |
| YUV | Color Space Pixel Format |
| YUV3 | CCIR 601 2:1:1 Bitmap |
| YWP | yWriter Project (Spacejock Software) |
| YYC | Wave Sapphire Distribution File |
| YZ | YAC Compressed Archive File |
| YZ1 | DeepFreezer Compressed Archive (BinaryTechnology) |