bug fix , serve existing file before download...

crmcache.go

 	id := attachmentid[0]
 	path := crmcacheFileName(id)
 
+	if isFileExist(path) {
+		http.ServeFile(w, r, path)
+		return
+	}
 	err = crmDownloadAttachmentAs(id, path)
 
 	if err != nil {
 		fmt.Fprintf(w, "reference number is not right")
 	}
 
-	if isFileExist(path) {
-		http.ServeFile(w, r, path)
-		return
-	}
-
 	w.WriteHeader(http.StatusInternalServerError)
 	fmt.Fprintf(w, "cannot build reference cache")
 }