如何用变量contentType实现R闪亮的下载处理程序?

docs ,下载处理程序应接受contentType作为字符串。因为contentType不是常量,所以我试图传递一个变量或reactive。但只要contentType不是严格意义上的字符串,应用就会失败。如何传递变量值?

我试过了 function(){} , reactive , eventReactive filename = function(){} .

ui <- fluidPage(
  downloadLink("downloadData", "Download")
)

server <- function(input, output) {
  # Our dataset
  data <- mtcars

  output$downloadData <- downloadHandler(
    filename = function() {
      paste("data-", Sys.Date(), ".csv", sep="")
    },
    content = function(file) {
      write.csv(data, file)
    },
    contentType = function() {
      "text/csv"
    }
  )
}

shinyApp(ui, server)

is.na() applied to non-(list or vector) of type 'closure' server error ).