It doesn't know about etf (yet - updates welcomed), but one place to look up unrecognised file extensions is
http://www.nationalarchives.gov.uk/PRONOM. If it has infomration of the fiel type it will also give you info on programs will read it (and to what extnetn they'll render the original correctly), when those programs are due to become unsupported and all sorts of bits and pieces (I shoudl perhaps say that I look after the back-end database for this, but since it's a government thing, I don't directly gain anything from advertising it).