The Collaborative International Dictionary of English v.0.48:

attache case \attache case\, attach'e case \attach['e] case\n.
   A flat, rectangular, usually rigid briefcase commonly used to
   carry documents for business purposes.
   [PJC]

WordNet (r) 3.0 (2006):

attache case
    n 1: a shallow and rectangular briefcase [syn: attache case,
         attache]