[NTG-context] escaping \ in Lua

Dear list, I have the following source: \setupinteraction[state=start, display=new] \enabledirectives[references.border=darkgreen] \starttext \startTEXpage[offset=1dk] \attachment[method=hidden, file=a(b).pdf] \stopTEXpage \stoptext The name in EmbeddedFiles is "a\(b\).pdf". Sorry for the obscure question, but how can I access this with "\goto{link to external}[a(b).pdf]"? I guess there may be some Lua magic, but I cannot write a proper ":gsub("%(", "%\%("))". Many thanks for your help, Pablo