Add download buttons in DT::renderDataTable

As Stephan said in comment, the way to add buttons is the following:

output$mytable1  <- DT::renderDataTable(
                        DT::datatable(
                            { plots.dfs()[[1]] },

                            extensions = 'Buttons',

                            options = list(
                                paging = TRUE,
                                searching = TRUE,
                                fixedColumns = TRUE,
                                autoWidth = TRUE,
                                ordering = TRUE,
                                dom = 'tB',
                                buttons = c('copy', 'csv', 'excel')
                            ),

                            class = "display"
                       ))