這個?
http://filext.com/detaillist.php?extdetail=GIExtension: GI  
Program and/or Extension Function  Company  
Audio Utility Tuningwrench Compressed File
Specific Notes  
(None) 
MIME Type  File Classification  Associated Links  
   None  
Identifying Characters  
(None or Unknown) 
Program ID  
(None or Unknown) 
General Notes  
Note: This entry has not yet been verified by FILExt. It was submitted to the list and appeared to be accurate; however, no verification links have so far been found on the Internet. Please use this association data with that in mind.   
This is record 5044 last modified on 2003-01-14.    
Extension: GI  
Program and/or Extension Function  Company  
Prassi or PrimoDVD Image
Specific Notes  
(None) 
MIME Type  File Classification  Associated Links  
   IsoBuster Data Recovery Tool  
Identifying Characters  
(None or Unknown) 
Program ID  
(None or Unknown) 
General Notes  
(None)   
This is record 15284 last modified on 2003-10-10.    
Extension: GI  
Program and/or Extension Function  Company  
RecordNow MAX CD Image  Stomp  
Specific Notes  
CD burning software. 
MIME Type  File Classification  Associated Links  
   None  
Identifying Characters  
(None or Unknown) 
Program ID  
(None or Unknown) 
General Notes  
(None)   
This is record 16298 last modified on 2004-03-23.    
Extension: GI  
Program and/or Extension Function  Company  
Sony CD Extreme Global Image File
Specific Notes  
(None) 
MIME Type  File Classification  Associated Links  
   None  
Identifying Characters  
(None or Unknown) 
Program ID  
(None or Unknown) 
General Notes  
(None)   
This is record 5045 last modified on 2003-01-14.    
General Info
If the extension you are looking for is of the form .??_ the file is likely a compressed file and part of an install routine. When the install program runs it will expand the file and replace the underscore with the proper letter for the file type. 
If the file is sent as an audio file with any three-character extension it may be a dictation file where the extension is the dictator's initials; playable by BCB Voice Systems software (see ??? extension). The file could likewise be a dictation file in TrueSpeech format. Use Windows Sound Recorder and/or Media Player to play these back. 
If the extension you are looking for has a number at the end or is only a number and there were no results consider entering the same extension format using zero or one as some programs produce numbered split files but when the split can be extensive (e.g., .000-.999) only the first entry is in the database.