Meaning "to fasten, affix, connect," which probably is the original sense etymologically, is attested in English from c. 1400. Related: Attached; attaching.
also re-attach, "attach anew or again," in any sense, c. 1600 originally in legalese and now obsolete in that sense; see re- "back, again" + attach (v.). The general sense of "to attach again" is by 1813 and might be a new formation. Related: Reattached; reattaching; reattachment (1570s in the legal sense).