added cache handling function

server.go

 func crmAttachmentHandler(w http.ResponseWriter, r *http.Request) {
 	fileID := getCrmFileID(r.URL.Path)
 	saveAs := GlobalPath.CRMAttachment + "crm_attach_" + fileID //add prefix for easy deleting
+	if isFileExist(saveAs) {
+		http.ServeFile(w, r, saveAs)
+		return
+	}
 	if fileID == "" {
 		log.Println("invalid fileID")
 		response404Handler(w)