set supporting file as xxxx

knitr_cache_dir <- function(file, pandoc_to) {
  ## paste(tools::file_path_sans_ext(file), "_cache/", pandoc_to, "/", sep = "")
  ## set supporting file as xxxx
  ## instead of the original xxxx_files
  ## in the 'graphics' file
  paste(tools::file_path_sans_ext(file), sep = "")
}