The Collaborative International Dictionary of English v.0.48:

Filefish \File"fish`\, n. (Zool.)
   Any plectognath fish of the genera Monacanthus, Alutera,
   balistes, and allied genera; -- so called on account of the
   roughly granulated skin, which is sometimes used in place of
   sandpaper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

filefish
    n 1: narrow flattened warm-water fishes with leathery skin and a
         long file-like dorsal spine