onTransferBoxStateChange

local started=false

addEventHandler ( "onTransferBoxStateChange", localPlayer,function(current,total)
  cancelEvent() -- disable current transfer box
  if(not previous or previous~=current)then
    if(not started and current~=total)then
      onStartDownload()
      started=true
    end
    if(finish~=total)then
      onDownload(current,total)
      if(current==total)then
        onFinishDownloading(current,total)
        started=false
        finish=total
      end
    end
  end
  local previous=current
end)

function onStartDownload(current,total)
  outputChatBox("START DOWNLOADING")
end
function onDownload(current,total)
  outputChatBox(inspect{"Downloading: ",current,"/",total})
end
function onFinishDownloading(current,total)
  outputChatBox("FINISH DOWNLOADING")
end

--[[
example output with script contain 15 files, size of this files: 8 921 880 B ( by windows ):
START DOWNLOADING
{ "Downloading: ", 0, "/", 8921880 }
{ "Downloading: ", 446094, "/", 8921880 }
{ "Downloading: ", 669141, "/", 8921880 }
{ "Downloading: ", 1338282, "/", 8921880 }
{ "Downloading: ", 1784376, "/", 8921880 }
{ "Downloading: ", 3568752, "/", 8921880 }
{ "Downloading: ", 8921880, "/", 8921880 }
FINISH DOWNLOADING
]]