这里有一种方法可以做到这一点，使用renderUI和uiOutput的组合来动态显示/隐藏第三个fileInput。

library(shinydashboard)
library(shiny)

ui <- dashboardPage(
  dashboardHeader(title = "Basic dashboard"),
  dashboardSidebar(),
  dashboardBody(
    fluidRow(column(6, fileInput("classe1",label="Class 1",
              multiple = TRUE, accept = c(".jpg", ".jpeg",".png",".tif"))),
      column(6, actionButton('add', 'Add a new input'))
    ),
      fluidRow(column(6, fileInput("classe2",label="Class 2",multiple = TRUE, 
              accept = c(".jpg", ".jpeg",".png",".tif"))),
            column(6, actionButton('clear', 'Clear')), 
          ),
          br(), 
          uiOutput('new_button')
    )
)

server <- function(input, output) {
  observeEvent(input$add, {
    output$new_button <- renderUI({
      fileInput("classe3",label="Class 3",multiple = TRUE, 
                accept = c(".jpg", ".jpeg",".png",".tif"))
    })
  })
  observeEvent(input$clear, {
    output$new_button <- renderUI({})
  })
}

shinyApp(ui, server)