Yes Mac Army Knife will do the job. You'd choose the File Attribute Editor, drag & drop your file into this program window. Then change the type & creator. Type should be MPG3 & for the Creator for iTunes it should be Hook. Press Update.
Other fixes can be seen here (see link to Stief's suggestion!!!):
http://www.gnutellaforums.com/showth...threadid=29223
This is fine for mp3's. But for other types of files, you'll need to drag & drop healthy files into Mac Army Knife to see what creator & file-type they should be & use those. eg: for Sit files: Creator would be SIT! & type will vary depend on version of StuffIt: eg: sit4 or 5 or the newer SITX for that type of sit only. Type version for SIT files is not too important (except for sitx.)
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cannot open anything (MacArmy Knife settings - click on link)