Display link to file on fileserver in shiny

I would like create in a shiny application a table with links to files on our file server. However it seems like links to the file system are not clickable.
If you download the page as html the link becomes clickable and the url resolves as expected. It looks like shiny is blocking urls starting with "file://///" ?

A minimal example below

library(shiny)

ui <- fluidPage(
          HTML("<a target = 'blank' href='file://///filesytem/filename.pdf'> link to file is not clickable </a> <br/>
                <a target = 'blank' href='https://community.rstudio.com'> link to website is clickable  </a>")    
        )

shinyApp(ui = ui, server = function(input, output) {})