The Collaborative International Dictionary of English v.0.48:

Attach'e \At`ta*ch['e]"\, n. [F., p. p. of attacher. See
   Attach, v. t.]
   One attached to another person or thing, as a part of a suite
   or staff. Specifically: One attached to an embassy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

attache
    n 1: a specialist assigned to the staff of a diplomatic mission
    2: a shallow and rectangular briefcase [syn: attache case,
       attache]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "attache":
   Admirable Crichton, adept, ambassador, ambassadress,
   apostolic delegate, artisan, artist, authority, career diplomat,
   chancellor, charge, commercial attache, connaisseur, connoisseur,
   consul, consul general, consular agent, consultant, cordon bleu,
   crack shot, craftsman, dead shot, diplomat, diplomatic,
   diplomatic agent, diplomatist, elder statesman, emissary, envoy,
   envoy extraordinary, experienced hand, expert, expert consultant,
   foreign service officer, graduate, handy man, internuncio,
   journeyman, legate, marksman, military attache, minister,
   minister plenipotentiary, minister resident, no slouch, nuncio,
   plenipotentiary, politician, pro, professional, professor,
   proficient, resident, savant, secretary of legation, shark, sharp,
   statesman, technical adviser, technician, vice-consul,
   vice-legate



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ATTACHE'. Connected with, attached to. This word is used to signify those
persons who are attached to a foreign legation. An attache is a public
minister within the meaning of the Act of April 30, 1790, s. 37, 1 Story's
L. U. S. 89, which protects from violence "the person of an ambassador or
other public minister." 1 Bald. 240 Vide 2 W. C. C. R. 205; 4 W. C. C. R.
531; 1 Dall. 117; 1 W. C. C. R. 232; 4 Dall. 321. Vide Ambassador; Consul;
Envoy; Minister.